If fixed CP is the final decision. It's not going to be the end of the world, and may in fact be a better choice.
We know that in leveling your avatar, Pokemon shares Ingress' trait of being highly exponential. E.g. a level 16 player in Ingress might be only four times the level of a level 4 player, but they have gotten there by earning 570 times the XP. The XP required for leveling goes up rapidly each level.
We also know from Niantic's statements that higher CP Pokemon will be catchable as you level up your character. What we're seeing right now from the testers is rapid level gain and the Pokemon they can catch also increasing rapidly in CP. But this is temporary. I suspect this game is going be like Ingress, level 1-7 are the warm up levels. However long you put in to get to level 7 though (a month?), you're going to need to put all of that time again to get to level 8 and four times that amount to get to 9. Once testers leave the earlier levels, the CP "inflation" will fade away.
I don't think there will be any chanting of "Pokemon are resonators" from players once they've been on level 10 for 5 months and have had the same Pokemon the entire time. I don't know about you, but I "bonded" with my Pokemon in Blue as a 9 year old within about 2 hours of gameplay, so we'll see how it feels when we're relying on our same friends for months.
Because of the wide variation we're seeing in Pokemon, that excellent Blastoise you evolved when you were just level 8 may well still be excellent a year later at level 11, months and months later. I think the "turnover" of pokemon is a little skewed right now and perhaps a little overblown.
The CP "fixing" might also lend to the uniqueness of Pokemon. Imagine how lucky you would feel when you score a high CP Pokemon in a type you've been looking for AND it has a nice fast move type and charge move type combination that you've never had. Any wild encounter could be a real treasure as the beta stands right now. I think I can see what Niantic has made this decision. Any encounter, anywhere could be a big win, there's a reason to go out and explore the world.
You still do have the power to evolve Pokemon and testers are suggesting that evolving a Pokemon generates something much more powerful than your average catch. This, combined with the "slow down" as you hit higher trainer levels are going to keep old favorites by your side for a long time.
Fixed CP also creates the opportunity for Niantic to reward participants in the inevitable "anomaly" events with more than just the typical legendary Pokemon. Naintic will run out of Anomaly awards quickly if its just handing out the 3 birds, mew and mewtwo. But if they generate a special high CP pickachu with a moveset that can't be caught in the wild... that would be just as unique as any legendary bird in the beta.