Interesting perspective: there's no "hard" barriers, but there may be soft barriers, especially if there's no Level-scaling mechanic.I think it's likely that I'll progress through all three routes at a similar pace. Since there probably won't be any level scaling it makes the most sense to me to just do whatever is appropriate for my level at the time. (besides, if one of the routes involves climbing to the top of one of those towers, it doesn't really make much sense to ignore it just because you're heading to challenge a gym)
I'd guess that the way the routes will be set up, it'll be possible (and probably sort of expected) for you to dip into them all at the same time.
In theory the whole "do whatever you want" sounds cool but in practice there will probably be a path through the region they expect you to take....and aside from maybe taking a detour to pick up a Pokemon I want for my team I'll probably just end up rolling with whatever makes sense at the time.