Yeah, I experienced the same with my Fenrir mount. I was grinding the 1 mio MGP out back in early HW and then they added the challenge log entries, glamour report and whatever and now people are swimming in MGP and the Fenrir has lost all it's prestige.

If you want rewards that last then you have to look at achievment rewards. If it's tied to an achievment then it likely will not get nerfed. Like the mounts from the hunt, some titles etc.