As a sensible notion I'd agree but as say us becoming a serious league challenger or trophy winner I'd bet my bollocks we wouldn't win a thing.
The fact is Schneiderlin for example is available and not mediocre so too Otamendi. I would be speechless if we started season the same bar a RB and a no doubt lesser keeper.
Regarding those specific names:
I couldn't classify Otamendi as mediocre. He's coming off a season where he made the La Liga XI over Godin and Sergio Ramos.
Schneiderlin is a tougher one. He's not mediocre but he could be a Young or Fellaini-esque signing in being a solid but limited player who gives us his all but isn't quite good enough to start for a CL or title contender. He's a solid player who is well suited for tough scraps, can pass the ball and 25 isn't old for a more defensive CM but there's no guarantee he'll step up comfortably and he may cost enough that we could sign, for example, any non-Bayern Bundesliga CM (this board seems to rate Xhaka) for the money and have some left over.
Our hierarchy of needs looks like this:
1. Have to get someone in - We've got a crisis at RB and GK since Van Gaal doesn't rate Rafael and De Gea will be at Madrid. Those positions need replacements, desperately. I don't trust Valdes to deal with crosses in the Prem or stay healthy and Valencia (or Young moving there) is a good backup option but no more. If we have to reach for an imperfect
2. Really need to pull the trigger unless there's nothing out there - Then, since Van Gaal won't play Di Maria at all centrally, it seems, we could really use a CM/DM to go with Herrera and then the 3rd spot can depend on opponent, fit and the form of Mata, Carrick and Blind, who are all strong players. Fellaini, Rooney, Januzaj and Pereira are also around.
3. Look for the right signing but with urgency - LCB is similar but a bit less of a need as either Jones (23) and Rojo (25 but new to Prem) could surprise us and emerge as a starter next to Smalling. We likely need to upgrade here but if we have to wait a year there is still hope of someone having a strong season.
4. Look to sign if we have an option we like - 2nd CM/DM to go with Herrera and the CM we sign. Again, though, if we mix and match right, then between Di Maria, Mata, Carrick and Blind there's talent here.
5. Only if the right signing is there - RWF or Striker. Get some more pace in the attack. I think Rooney will score goals if he's just chucked up front as a 9 all season, so I'm not as concerned with the 9 spot as others. RVP and Chicharito will score goals off the bench too and Wilson or Henriquez can fight to be the 3rd striker. If we sign a player here it should really be someone we think has incredible upside or a replacement for the above names.
If Di Maria does go, I think RWF becomes a position of need.