What's the difference between level 100, and level 90? What's the difference between level 90 and level 80? What's the difference between level 80 and level 70? If you can answer what separates 30 levels, then you have very well made your point. Reducing the level cap is very possible, the same way they reduce it whenever you have overleveled Pokemon participate in level 50 tournaments. If you trade a Pokemon that's level 100, it'll just be level 70. And heck, if you think that's too much of a stretch, I'll go so far as to bump it up to 80 then, because let's be frank, there is really no difference been 80 and 100, which you haven't stated in your post so far.
Was that hard work all for naught? I'd argue not, since most (I daresay a good majority) of people complete their EV training well before level 50, let alone anything past that. When you actually think about it, back in the day, level 100 seemed like something to strive for. Obviously I can't speak for anybody, but I don't see that going very well for myself. Personally, I have grinded more in my Black 2 than I have in any other Pokemon game since Emerald Version. The highest leved Pokemon I have, I think is level 75-78, and that was because of the ridiculous amount of grinding I had to do because I was a) bored, b) mostly for side-quest purposes, like for challenging Benga and other extremely high-leveled trainers.
But after you beat those high-leveld trainers? Where's the satisfaction? After you pat yourself on the back for going to level 100, the only thing left is..."well, what now?"
You also have to take into consideration a lot of Pokemon's experience gains, as you're very well aware that not all Pokemon gain experience at the same rate. How long would it take a Pokemon like Hydreigon to reach level 100, as opposed to Altaria? Unlike Hydreigon, Atlaria doesn't require as many experience points to reach level 100, while Hydreigon needs a ridiculous amount, and I know from personal experience because I know how frustrating it was just to evolve my Zweilous all the way to a Hydreigon at what, level 64?
You have a good point that level 100 is definitely something to shoot for, but I believe that, for those like myself, it has lost value. If there was certain incentives to shoot for as far as leveling up your Pokemon to level 100, then by all means, I could care less. But it just seems like a gigantic waste of time when you're done beating pretty much the strongest trainers in the game.