I doubt there will be ten generations of pokemon, from what it looks like the designers are running out of ideas. There already making you use pokemon from other generations just to catch a new pokemon, also making new evolutions of pokemon from the other generations.
You mean like how they made new evolutions for old Pokemon in 2nd gen?
Anyways, yeah, what TBA said is basically my thoughts on it. Even with more Pokemon included and online trading, things haven't changed too much. You still need two versions and two people trading back and forth, regardless of how much longer it takes. Even in gen 1 you couldn't have completed your 'Dex without that much. The same can be said about the unobtainable Pokemon, because how many of us actually had legit Mews way back when?
That said, since 3rd generation fully completing your 'dex depends heavily on which Pokemon games you have access to (ie. you needed FR/LG/Colosseum to collect everything minus the unobtainables). In D/P, if you don't own a majority of the previous games or know someone who does, you can't get them all. Is it reasonable to expect people to buy most of the games? Honestly... if this was any other gaming series, it wouldn't be. Maybe the majority of us do own multiple games from the Advance generation, but that doesn't really make it "reasonable" - it just means we're all obsessed with the series enough to do it. You can't really expect a casual fan of Pokemon to go to the same extent a hardcore fan would.
Take for example as well that some people ONLY got into Pokemon with the release of D/P (or they got back into it and don't own any of the GBA games). For them without WiFi, there's no way they can complete their PokeDex, and it's not reasonable to expect them to back-track and buy the GBA games.
But why am I even bothering? We all know the "gotta catch 'em all" slogan was created solely to promote and sell the games. If you're a hardcore fan, you'll find a way to complete your PokeDex one way or another - even if it means buying yourself a second DS. If you're not... you just don't care about it. Even a lot of us who've been involved with the franchise since gen 1 don't bother going to the lengths of capturing every single Pokemon each generation (I know I don't).
Summed up, yeah, it's reasonable for the people who actually care to "catch 'em all" because, even though time-consuming, it can be done. For casual fans, it's not really as reasonable, but it's not necessary either. Catching all of the Pokemon is like beating the super-powered secret bosses in an RPG... You'll do it if you want it enough, but it's not needed to beat the main game.