There’s a big difference in a quite simple aircraft like the Mig-21 and this proposed 5th Gen fighter or even current Su-33s or 35s. For one thing, the Mig-21 was manufactured all over the world, you don’t have to even rely on Russia for support. The Israelis for instance have been refurbing and substantially updating those aircraft for many years.

And poor Russian support isn’t related to payment, it’s related to poor support period.