Don't see a problem with us continuing to try and loan Hendo each season provided it continues to work for all parties and we aren't getting done over financially.
Even if we signed him permanently, if Man Utd or another real top top club came in for him and offered him real money then he's gunna leave SUFC at that point anyway so what's the difference if we try to loan him from season to season? The only real value I would see is that we would get the future transfer fee, but against that is the argument that if we tried to get him now it would cost us all or at least a very large chunk of our summer transfer budget.
As long as we have a decent lad in reserve, what's the issue in trying to keep loaning the lad? We get to play him whilst he's here and we get to use that money saved on his transfer fee, elsewhere on the team.