"Power Creep" means that the latest powers are generally better than the older powers. In which case, people just swap their old powers for the new shinies and don't look back.
The fact that he wants both his old powers and the new powers indicates:
1) that the former and the latter are roughly equal in power and
2)that he wants his character to be more versatile.
2) is just a personal preference, and 1) means that there's little to no power creep.
At least, that's what logic seems to dictate in this instance.
It'd be interesting if you could share your POV in this case, because I'm not seeing how you're going from "someone would like to have more At-Wills" to "Power creep is steadily increasing".
I think the logic was assuming that an encounter power is better than an at-will, so Celebrim might have assumed that the at-wills are stronger than the encounter power.
But that is not what Kzach described. he wanted to exchange a power he could use once per encounter against a power that he could use at-will. The at-will power might be, in an individual round, still be weaker as the encounter power, but he can use the at-will power again. It's about exchanging versatility for power. At least that's the ideal. But it doesn't work that way in reality. The ability to use the same power three times in an encounter might be more important than the ability to use a stronger power once per encounter.
That's ultimiately why all classes have the same base number of at-will, encounter and daily powers. Changing the numbers makes things more complicated.
In a short encounter, encounter powers probably have more impact. In an extended one, it lessens and the right at-will choice is more crucial.
During a short "adventuring day" with only one or two encounters, daily powers will have a notable impact. If the adventuring day consists of a dozen encounters, daily powers will have less impact and encounter powers (and at-wills, and action point benefits) become more important.
If you can keep the number of rounds in an encounter and/or the number of encounters per day constant (or know the standard deviation), you can balance different number of encounter/daily powers per class. If you don't keep it constant, you can't.