The rules haven't been changed, Pykey.
I agree with you that the MGP is an entirely different race, and while it could be seen as a stepping stone to the TT, I don't think it should be a path back from it, or a place where failed TT entrants go. Lets face it, if someone has a couple of goes at the TT, and can't qualify, they probably aren't fast enough for the Manx either.
As far as the "training for the TT" bit goes, yes, that's great, but some of the best MGP riders have retired after winning, because they simply don't want to do the TT. If you win a Manx, it is assumed that when you move to the TT you will weant to win there, but there is little chance of anyone wining the TT who does not have a huge budget and a team behind them.That may not be their style.
It was always the way, and back in the 20s that's why the Amateur TT, later the MGP came about. Lets keep it so people have a choice, and keep them separate and different.Diversity rules!