Paris never struck me as experienced or grown up enough to be the captain's right-hand man. Although I guess you are short on suitable candidates.
You're right to a point and that sort of tension is actually in part why I feel like bumping Tuvok and Chakotay off the ship and letting Paris be the XO (especially keeping him at Full LT to earn LT Cmdr) would have been especially great.
Chakotay himself said it best when he said Paris was something of Janeway's reclamation project, well, one of three (7 of 9 was easily another and one could argue Toress was also, after a fashion).
Being a little less mature assuming the role would have given a lot of room to grow and a little room to fall.
For him to fall the way he did in the series, they had to invent an entire scenario presented in and taking affect in a single episode (in "30 Days"). If he was the defacto XO, a position he had great potential for but really wasn't QUITE ready for, that does line up with a whole bunch of "filling shoes" scenerios to give him a chance to slip and grow.
Heck, I almost hate to say it, but in doing it that way, Paris being able to lament about Tuvok and Chakotay NOT being around may have actually ultimately gave us more reasons for us to care about the characters as legacy characters then riding them to the end of the series.
Besides, I think we could have REALLY sent both Chakotay and Tuvok out in a REAL blaze of glory in a way that would have been a real tear jerker compared to the "Oh, no more Kes?...." especially in the Borg Resolution --- there were all kinds of good potential places to do it.
Besides, I look at the series as a whole think about how many more regulars we could have enriched the series with if we opened up the salary from three cast positions. We could have seen a lot more of Carey, Wildman, Vorik and we really could have and should have expanded the roster of regulars by a lot. With as small as Voyager was and the fact we weren't rotating crew EVER, I still say we should have had so many regulars that it felt like we met the whole ship.
Oooo --- its fun thinking about the what ifs
............. and Guy has a point --- as usual.