I think the Marian Hegemony, Rim Collection and New St Andrew actually still build Retrotech Mechs though only those. The Hegemony builds actual Aerospace fighjters and "modern" tanks. Though according to FM 3145 the Collection builds actual Battlemechs now (Locusts, Wasps and Stingers).
In a way they might have a use but more as Militamechs or perhaps GuardMechs for companies who don't want to buy modern Battlemechs. After all according to the lore you need less resoruces, the tech is less advanced so you don't need sophisticated factories or a highly trained work force