Here's the problem with that statement: There are thus no parts for a 'Mech design that's been out of production for years, so how come people keep digging them up and putting them back into service. Lack of parts interchangeability also means that I want as few designs as possible, so I'm only ever going to have one design for a certain role, if that.
It depends who you are.
If I'm Duke Jerkface in the year 3035, and I have 10 regiments of mechs, and have to decide what mechs to try and obtain, then I'm not going to focus on the Scorpion. If my men happen to stumble across a functional one, that's fine. But if I only have one in my entire force, at a certain point it gets easier to just scrap the thing and replace it. I'm not going to make any special effort to keep it going if there's a better replacement available.
On the other hand, let's say I'm Farmer Jones' son. One year we have a real dry spell, and the pond in the old back pasture starts to dry up. Oh look, there's a metal thing sticking up out of the dirt in what had been the bottom of the pond. We dig it up and find out it's a Scorpion battlemech. 100 years ago or more some guy failed a piloting roll, fell into the water, his cockpit breached, and he drowned. Now I've got a fully functional battlemech just as soon as I can replace the electronics inside (or maybe just dry them out if they're milspec). To me, that Scorpion represents extreme social mobility. I can go from some poor farmer's son, to a nobleman if things work out okay. But this is my only mech. No matter how bad it sucks, I'm going to do everything in my power to make it work again. Then I probably need to join a merc unit or something to legitimize my find.
Thinking of it that way, I'd say that as time goes by (and the tech recovery starts), the weird rare mechs are more likely to be heirlooms than to appear in regular House units. As soon as House Davion has an option to get rid of that last handful of Clints, they probably will. Scrap that sucker and replace it with another Enforcer or something. But to people where that's their only mech, it's worth any effort to keep it operational.