Blind has started every Premier League game he has has been fit for. Fellaini and Young have been ever-presents since March. Jones has played little to no less than any of our other centre half's. All of these players were regulars during our best run of form. If these aren't regular first team players then who is?
That you believe these players won't be in our first team next season adds weight to my point. We're reportedly looking for a right back, centre back, central midfielder and wide man (Depay) because we know that we can improve on these players. So, why would you advocate signing more squad players when it's clear we can fill the first team with the best players in said positions?
Both Fellaini and Young can make a difference from the bench, they have done this season. And players like Blind and Jones, or Rojo, aren't on the bench to make an impact. They're on there because they ensure we have quality cover the first team. If Luke Shaw gets injured them we have a very capable alternative in Blind, for example.
Who would you sell, out of curiosity?
Like i said, if all our players are fit and performing at the level they are expected to perform then none of those players would make the first team next season(Assuming we sign a RB this summer), Di Maria wasn't a regular during our best run of form is that going to make him a benchwarmer next season? we all expect him to be World Class next season which would make him the first player on the team sheet, even if his performances are still average, i still expect him to be a starter...just because.
Fellaini is the only solid squad player we have who would be able to make an impact from the bench, i think the mistake you're making is you're planning for next season with this season in mind, this was a weird season where our top players failed to perform and players like Young and Fellaini performed well but not well enough to warrant a first team position next season, Young was really good this season, but it still wasnt anything close to what a World Class Di Maria is capable of doing, with Di Maria and Depay in the first team next season there would be no need for Ashley Young to come off the bench as an attacker as he is basically Di Maria without the magic and the end product.
Having Jones,Rojo and Blind on the bench to cover the first team would be wasting a spot on the bench imo, they can all play at CB and 2 of them can play at LB, so it would make more sense to give a spot on the bench to a STRONG utility player.
I would sell RVP,Rafael,Jones and Hernandez and loan out Januzaj and Wilson.