She wasn't "written into a corner," she had some of the best potential of any of the characters. If they were going to keep her around, they simply wouldn't have "powered her up" to the levels of "The Gift."
If they could write for Troi on TNG, they could easily write for Kes on VOY. Kes was like a more powerful and more interesting version of Troi.
No, she was not "written into a corner" in any way. That's just one of the lame excuses they tried to come up with to justify their actions. In fact, if you look at season 3, I get the impression that Kes was becoming a more important member of the crew.
However, I must state that the somewhat ridiculous development of her "superpowers" in "The Gift" was a way to dump the character as soon as possible, just as the writer who wrote the story was ordered to do.
It's also ironic that they used Kes's "superpowers" as a way to get rid of her when they just had brought in another main character with "superpowers" who could fix everything with her Borg knowledge.
As for the comparision between Kes and Troi, I think that Kes was the more interesting of those two. Kes had more guts and were in many ways smarter than Troi. Mind you, I actually like Troi too but compared to Kes, she was a bit wimpy sometimes and not as clever. And Kes had at least the good taste not to end up in soppy relationships now and then.