I agree that 4's will continue to dominate for many years to come, but there are several other genres that regularly use 5's and 6's as part of that 'sound': Modern Gospel, R&B, Reggae, Soca, Merengue and several other Caribbean styles just to name a few, and that's just in my neck of the woods...
Not to play devil's advocate, but maybe the reason why they're so dominant is that most of the bass legends (and a large group of members on TB) were born into a virtually 4-string only world, and that subsequently became their comfort zone (this is NOT saying Vic or Marcus or Stanley etc. couldn't kill on a 5 or 6, but it'd be a change of pace from the norm i'm sure). Not to mention you couldn't walk into most studios without a 4-string Fender until the last what...20 or so years? (could be wrong, someone correct me on this)
As for the younger generation like myself, IMO we were either A) was taught by the older crowd, who encouraged learning on 4's first (again, not necessarily a bad thing IMO)...B) Learning about the same Bass legends and in an attempt to emulate them, buys a 4 banger...C) Heading to the nearest music store and noticed an abundance of 4's...which offers more variety/seems a little less daunting than 5's to the newbie.
For all these reasons I believe the whole "crappy B string/tonally different" argument is moot when faced with such odds in favor of 4's before that phase is reached (I mean, how many of us were truly on the epic quest for Tone we suffer from today

the first time we picked up a bass?). Don't get me wrong though: I love 4's, there's a bit of romanticism with the whole thing, and I agree if the style/songs you play don't require the low notes/extra position then use a 4. But I learned on a decent Ibanez 5 in high school so there's no 'getting used to it' issue...as one person said, moving from 5 to a 4 feels like you're flying.