The problem with that point of view is that overpaying Fellaini alone does not exist in a vacuum; it inevitably results in one of all of the following:
- Increased ticket prices
- Reduced investment in training facilities
- Reduced investment in stadia
- Reduced wage budget for playing staff
- Reduced total transfer capex
Personally every single one of those should take priority over overpaying a backup midfielder and the ramifications that has.
Therefore if you care about ticket prices, United's facilities, transfer budgets, wage budgets etc; then you de facto care about how the club is run and ergo care about overpaying Fellaini.