I agree with your point, partially, yet I think the main culprits for this kind of stagnation are actually ourselves. Sir Alex's over-reliance on experience in the latter years has came at cost to a few of our players. I look at Smalling and Jones and I get frustrated. I then look at Raphael Varane over at Madrid and it intensifies. At the European U21 championships we had the two best central defenders at that tournament in Smalling and Jones. Ours. Now? Neither are starting every week. Both of whom have been moved out of position and seldom played with any regularity. It's why I would have happily said goodbye to Rio not just last Summer, but even the one before last. As for Varane, who moved in the same Summer as Jones, he has played regularly for Madrid in the biggest of matches. Had Jones been at Madrid and Varane at United, I think we'd see a similar situation.
Smalling is now 24 and is not the footballer he should be. He certainly doesn't have the reputation his defensive skills merit. And that's a shame. Now, I know these are defenders so it's a bit different, but I see little point in blasting 30 million on Barkley unless he will play week in, week out. I honestly believe that Nani and Anderson were hampered by the steady reliance on both Giggs and Scholes. Anderson, more so, quite often played well yet found himself on the bench due to a rotation policy that only seemed to benefit about 3 players out of a squad of 25. Nani wasn't disrupted in fairness, as he's done quite well over time.
So we sign Barkley. What next? Is he going to benefit from playing once every three weeks in his best position in the way Welbeck, Smalling and Jones have done? Or, is he going to progress further by staying at Everton, playing in his best position regularly and biding his time until he can get a move to a top club that he will directly influence from the beginning, instead of being another one of those young players that stagnate due to bloated squads at the big clubs.
I think he'd be mad to move right now.