Nah man, you're overdramatising this. McTominay is an academy proffesional, not even a bought prospect. Fred was bought under Mourinho for 50m and instantly flopped. Since then they've both made themselves starters in midfield, and also starred in our second place finish. Our options until they started performing to a certain level were all worse, more expensive flops themselves and are nowhere to be seen right now. These guys are top 10 PL midfielders, theres nothing wrong with their mentality, so yes, they can clearly be strength in numbers as squad players at United right now. What you don't want in your backup midfielders is those that have no particular strengths or adaptability. McTominay can be used for his strength and physical assertiveness, while Fred can be deployed to harass and cause havoc. This is things that help winning midfield battles.
You are making the mistake of devaluating here, thinking they are worthless because United in this current predicament has overplayed them and asked more of them than what they're capable of, asking them to be something they're not. Arrogant with good faith, or an elaborate effort and excuse to not invest in midfield? Either way is probably fine, but blaming the lack of standard in this club on a couple of honest players actually giving it a go is stupid and covers up for the real issues, such as lack of investment in midfield.