alt Hacker NewsBattleTech is somewhat of a competitor, and a variety of smaller games have some niches.
Plenty of people use proxies, too. There's places that do monthly packs of new STLs that could be an entire faction army, and there's long been places that sold "definitely not Space Marines and Sisters of Battle" minis too.
They don't have a threat of anyone overtaking them at current, but AI making alternatives in this vein even cheaper could eat away at portions of their bottom line.
As a Battletech lover, the phrase "somewhat of a competitor" is a bit vague. I see Battletech as a 3%er - one of a few 3%ers - compared to the near-monopoly of WH40K (and fantasy WH).
As an aside, I am somewhat disappointed that Battletech's appeal to the mainstream is largely down to the Mechwarrior games which have minimal lore.
There is so much more that could be done. But the current owners seem to be pretty poor at translating all their paperwork stories for the modern crowd.