I agree it's getting really hard to find a rationale for keeping him. It seems increasingly clear he isn't going to do well either as a holding midfielder, or as a box-to-box player. And the No 10 role belongs to Bruno. That seems to require us to play 4-1-4-1 in order to utilise him well, but that simply doesn't appear feasible without a holding midfielder who can offer better protection than we have now, and/or a central defence less in need of protection.
While I agree it's a questionable approach to consistently re-sign players even if you plan to move them, I think that's probably a more justified approach with Pogba than with, say, Phil Jones. If he moves, it's unlikely it would be to a club with a much more restricted wage structure.