The biggest beauty of all Sartris is that if you want those restrictions are your table, do it. No one is stopping you. If you want old gear gone, don't use anything except this era's TRO mechs, the nuseen, and the new tech variants. You and your group have 100% control over the games and restrictions you have at your table. For example, our group banned the use of C3, TAG, alternate ammos, and a few other things because of how gamey they get.
For someone like me that loves so many points across the entirely of the universe's lifespan and loves using my imagination it have no hesitation or difficulty explaining why a House unit would want to produce a Star League Royal mech. Why would you just use a bunch of modern glass hammer XL engines and gyros? Why not go with a reliable zombie mech that has stood the test of time, both from 2800 to 3150 and 35 years in our time?
I could draw a parallel to the last season of Game of Thrones (SPOILER), one huge issue was where the army she attacked King's Landing with came from. Easy, a couple companies of Unsullied were holding Casterly rock, ports, and bridges between the North and South. Groups of Dothraki were patrolling South of Winterfell to ensure they were not sneaked up on by Cersei. They then rallied as Jon marched South. I have been chewed out multiple times online because I had to create that very easy to comprehend narrative rather than be told it in the show.
New players have it easy to join. They borrow units, explore sarna, check out IWM, they can play with other's mechs to start and get a feel for speed, lasers, missiles, autocannons and more. If they want, they can be like me, I purchase mainly because I like the look of a mini. The weaponry may play some part but I will field a mech I love the look of 9 times out of 10 and it is even better when there are multiple variants to choose from.
Like I said, family mechs, hordes of House and Clan mothballed mechs, Star league caches, retooled factories, etc. etc. all leads to the widest variety of mechs available in universe right now in 3150.