If you're not careful I think you could be putting yourself in the position Liverpool did a few years ago, where it gets harder and harder to get back into the Champions League the more time you spend out of it. Frankly, I think it's already happening. You signed three players this summer - two for big money - but they were all from British clubs who finished below you in the pyramid. I think Pogba was the last player you signed after going into battle against the other top clubs in Europe (maybe Fred, actually). Now whenever you go after that type of target they either stay at their current club (Eriksen, Dybala, Fernandes) or end up going somewhere else (De Ligt). Hell, even the CL-quality players in your squad have either agitated for a move in the past or eventually got the transfers they wanted (De Gea, Pogba, Sanchez, Lukaku, Herrera). I think the name "Manchester United" still means a lot in British football but I don't think it has anywhere near as much pull on the continent anymore. It's all well and good trying to sign players who'll get you back into Europe's elite but those players actually have to want to be part of the project, and the longer this period of stasis continues the harder it's going to be to convince anyone to join it. Putting a big-name manager - or even a remotely interesting up-and-coming tactics nerd - into your dugout might help matters, but I'm not sure what Pochettino or Allegri could really change at this point. I think they'd both get you back into the top four/six on a consistent basis but under this current regime I don't think they'd have the funds, the time, or the resources to do an effective job long-term. Your board are very reactive and impulsive when it comes to squad and staff management and until things change upstairs I can't see much changing on the pitch.