LAMs were not "going strong" by any means. Their numbers were in constant decline for centuries, and many operable LAMs suffered from shortages of parts and equipment that they simply could not replace. There's numerous mentions of LAMs being stuck in one mode or unable to properly transform because of missing or damaged components. The low-rate production from LexaTech and the warehouses were not keeping ahead of losses by any means.
Those mentions could be the proverbial interesting circumstance that gets a mention, and not the common case. Also, look at the changes in fiction as each new mention comes out and note what side of the Harmony Gold Suit line that stands.
So, a lot of the stuff you're mentioning is mostly speculation, and not hard fact. There definitely seems to be two biases regarding the proliferation of LAMs and their current activity. Until a sourcebook comes out singing 'The wicked LAM is dead' a la Wizard of Oz, and a Dev confirms it, I'm definitely exercising the 'it's your game' and 'What Colbosch said' policies.
Finally, I don't see how the Firestarter is a rare 'Mech. Yes, it's original factory was destroyed, but that was not until some point in the Third Succession War. Even then, Coventry Metal Works picked up production and continued to do such continuously until at least 3145. The Firestarter is frequently seen in units across all five Successor States, Mercenaries and the Periphery. Furthermore, unlike a LAM, it does not require specialised parts, and could easily accept components "donated" by another 'Mech. There's really no comparison between the two.
The Firestarter is said to be rare. It's only deployed at regimental levels and carefully so, because is so specialized, like another Mech series constantly talked about, or the Ostscout. #P Sure, every house has one. It may even be seen in Company or Battalion formations, especially for irregular style forces like mercenaries that decide they must have one. But, you don't see them fielded in groups. Not like a lot of other Mechs. A lance of Dragons or Panthers I won't bat an eye at. Nor a lance of Locusts, Stingers, or Wasps. But a full lance of Firestarters? Only in Mech Commander or other computer games.
And, depending on source, most Firestarters were actually being maintained from parts warehouses for a time. For me, I kinda like wrapping my head around the notion of trying to reconcile the new fact with the old fact. It's quite possible that many units with a Firestarter could only pull from parts warehouses, not being able to get fresh replacements from the nuFactory at all or right away.
It was the Succession Wars, after all.
That aside, a couple other things.
I've seen way too much in newer books that are supposed to be a reprint of the older source material keeping mechs alive that were slowly going the way of the dodo by suddenly bringing up some factory putting out new chassis somewhere. Honestly, they've shown that with enough work, there are plenty of garage shops that can assemble Mechs from scratch and keep a design alive. The Goliath is said to have some of those on various worlds. It doesn't have to be one of the large ones that seem to be the default.
Clan Tech is possible in the Inner Sphere, now that they've had a chance to backwards engineer it, and those weapons can be made the same way as a lost chassis. The only reason it hasn't taken on as mainstream output in the IS has so much to do with infrastructure. With the destruction of the Jihad and the reformations of the Republic, I imagine that old infrastructure can readily be upgraded post 3150.
How many old designs will have to undergo new upgrade refits or be scrapped then?