I agree that all of those players are decent, and Lindelof has improved in the last month or so, so I'm hopeful he will come good. The problem is decent players aren't good enough for a club the stature of United; I personally believe a full season at Villa ( esp. working with John Terry) will improve Axel enough defensively to give him a shot at United's first team, if not first XI. He is a big, strong lad (although he could improve his aerial ability), pacy enough for the Prem, and is good enough with the ball to build attacks from the back.
I can see Rojo and Bailly leaving in January or the summer, Jones is an injury nightmare with spotty form when he's healthy, and while I think Smalling is a solid defender, he is completely useless playing the ball and the game is quickly passing players of that mold by. Also, Smalling is getting close to a testimonial at United, which is something fans would likely see as representative of the decline since Ferguson retired, so he might be off as well before that comes to fruition. I can't see United selling Tuanzebe in the near future, especially if he continues his development at Villa Park and United eventually bin off a couple of deadwood defenders. He would be a great pickup for Aston Villa though, and if they pull off that signing I know a lot of United fans will be very disappointed.