I disagree that we need a "proven veteran coach" as much as I disagree we need an "inexperienced former legend coach" per se. Both options would fail without a sporting project. Without a plan, be it short or long term.
It all depends on your viewpoint, and this is mine:
A "proven veteran coach" at this point would be useful in order to prolong this Barca Era. Let's not kid kid ourselves, this Barca era will be officially over when Leo hangs up his boots or age finally catches up with him. We shouldn't expect major changes or any long term success following this road, not even so much better football. We should, although, expect better results at some important games and perhaps another CL title, as a last hurray, for this core.
An "inexperienced former legend coach" at this point would be out of his depth, status wise, even if he turns out to be a great tactician. He would need to make a huge revamp to the squad, letting off most of the seniors, as it will be very hard to impose himself as a Coach when dealing with friends. This would mean that the team would be in a rebuilding phase, and we should not have expectations for any titles for the next couple of years at least. And this is key.. the next couple of years is the time in which Messi is still here.
For me it boils down to this:
Are we prepared to waste this period, trophy wise, hoping that in a couple of years time this team will rise again to the standards it once had. Or should we try and utilize an aging, but World Class or even once in a lifetime, core of players, pair them with some hungry and talented youngsters, in order to extend our trophy cabinet and cement our status.
I'd go with the second option. Even if Xavi was now ready take over, which he isn't, he would be the wrong choice at this point in time.