As I’ve mentioned before, if you go full or even mostly LRM, you lose a lot of the potential of an Assault ‘Mech chassis. An LRM platform wants to stay back or ideally out of sight entirely and bombard the enemy without the enemy being able to respond. But an Assault Mech is meant to soak up damage.
They are the platonic ideal of the Distraction Carnifex ploy, because if you ignore them to hit more fragile targets, they’re closing in on you the whole time, punishing you for ignoring them, but if you focus fire on them, they just eat up the damage and ask for more.
You're confusing LRM platform with indirect fire support platform. LRMs can be used for indirect fire support, but they can also be used as damage dealers in plain sight. The ability to stand in the open substantially improves effect per LRM tube due to improved accuracy.
An LRM-20 with SHS does an average of 12.694 damage per hit for 18 tons (12 shots). A PPC with SHS does 10 damage per hit for 17 tons.
An LRM-20 with DHS still does 12.694 damage per hit now for 15 tons (12 shots). An ERPPC with DHS does 10 damage per hit for 14.5 tons assuming you value the half of the eighth freezer it doesn't need as a half ton saved. You can get more heat included medium lasers with the ERPPC, but you can get two of them with the LRM-20. An LB-10X with both ammo types does 10 or 6.482 damage for 14 tons. An ERLL does 8 damage for 11 tons. A gauss rifle does 15 damage in 17.5 tons, again counting a half used DHS as half a ton.
I think you didn't have Artemis IV, but that would boost the LRM-20 to 16.25 for 16 tons.
LRMs are competitive in damage per ton with the advanced long range weapons even without advanced technology apart from DHS. And that's in a direct fire role. Indirect fire opportunities are pure bonus. So long as there are any limits on advanced weapon availability LRMs
You're also ignoring the size of your threat bubble. Going from PPCs or LB-10Xs to LRMs increases your threat bubble by as much as going from putting your LB-10X on a Longbow to putting your LB-10X on a Clint. And when a Clint moves to hit the far edge of its threat radius in one direction it reduces its threat radius in the other. If you base your threat bubble on long range weapons you can't be drawn out of position.
The Longbow ... doesn’t fill this role. It vomits LRMs in slightly heavier amounts than the Archer does, sure. But, uh. It’s actually pretty underwhelming in other ways. If you want a fifty LRM throw weight, you can get there with a tank much more easily, much more cheaply, and without occupying a slot in your assault Lance that can be better filled by, say, an Awesome or a Battlemaster.
You're not thinking about how to make the Longbow fill the role because you seem to have a blind spot for direct firing LRMs. You use Longbows exactly like you use Awesomes. You stick them in plain sight on or partially behind a ridge and make the enemy choose between being hammered by long range fire or letting themselves be enfiladed by your battle line trying to get under their minimum ranges. You try that with LRM carriers behind the ridge, but you risk the enemy either not backtracing the LRMs in the heat of battle or killing your tanks before you can reinforce if they're a little more successful in pressing home the charge than you'd like. More short range backup weapons would be nice but it's nothing that can't be solved by sticking a couple Demolishers on the reverse slope.
You're also comparing to the Archer's alpha strike not its sustained damage output. The Archer can not consistently put 40 LRMs downrange because it has 12 heat worth of LRM tubes but only 10 single heatsinks. If it's standing still it can put out an average of 35 per round and if it wants to move it drops to 30 or 25. The Longbow is still putting out 50 LRMs every turn until the bins run dry. When you take cooling into account and compare within the same tech level you can see that the Longbow is getting quite a bit in trade for its assault mech speed even if some of it is wasted on excessive cooling.
I also I am thinking in terms of a Lance force structure. The Commonwealth loves the Zeus. It has an enduring love affair with the design, likely because Defiance produces them in greater numbers than any other Assault ‘Mech due to the combination of two lines and their relatively light weight (also, because they’re fucking DefHes, and the only way you beat their production numbers is by cheating.)
Not really applicable to the longbow discussion, but you're thinking in terms of weight not movement. It is not sensible to deploy Zeuses in assault lances unless you have a shortage of slow assault mechs. With their movement profile they should be either leading heavy lances or mixed with Battlemasters and old style Banshees in assault-in-name-only lances.
The other sort of issue is that with the improved technology that’s becoming available, LRMs aren’t as efficient as the more advanced weapons. I’m not saying they don’t have their niche, because they absolutely do, but the ERPPC/ML meta is real.
Looking at the numbers above they're more efficient than most of the advanced weapons. Catachan is having trouble with the Gauss line so those aren't on the table. The ERLL is more efficient. So would be an old style PPC, but both of those fall behind in range.
The old Longbow has an issue with inadequate medium lasers, but so long as you bodyguard them with something like a hunchback per lance they're entirely competent to stand on a hill and make the enemy pay attention to them and unlike tanks survive that attention without having to be towed to a repair depot. With just DHS and CASE the Longbow could have quadrupled its medium laser armament with tonnage to spare without touching its LRMs at all. This might even be too much short range firepower as if an enemy realizes there's no advantage to be had at short range he might refuse to take the bait.
Sorry to break it to you, but it’s a 45-tonner. It is fragile by definition.
An XLFE means it can be made sufficiently killy that it is much more likely to do unto the enemy before the enemy can do unto it. No point in building a bruiser until you’re into the Heavyweight bracket.
That said, an LB-10X to make life interesting for things fast enough to play keep away from the Choppa would be a solid improvement. A 45 ton ‘Mech that can headcap. How horrifying.
There's a spectrum between a bruiser and something that will disintegrate in a stiff breeze. Just because a thief can't wear full plate is no reason to leave him in street clothes and use constitution as a dump stat. Sometimes he won't kill his target with a single backstab or one of his target's buddies will decide to go after him not the fighter. There is no such thing as killy enough to not need to maximize survivability unless you're fighting under Clan dueling rules. Not at least until you get into something like CannonShop's nuke spam retrotech warships.
What there's no sense in is mounting a hatchet on anything slower than 5/8 or anything without TSM.