Very generous of you to bracket Warnock alongside Bassett, (4 promotion less seasons in the championship was it, promoted with a decent budget and instant relegation back down again). The promotion and triple assault season were good. The rest deeply meh or worse.
I do think managers have a shelf life. The only reason long term managers seem to have better results is that the poor managers in football get sacked quickly.
You can make a compelling case for better ownership and better overall sporting direction (technical director other staff etc) being more important to success than the manager/coach.
I, like many others on this thread, think its long overdue time for Utd to substantially invest in the sporting direction of the club both in staff terms (technical director/analysis/scouting) and infrastructure (training ground/ground upgrades).
We need a modern coaching set up with a gifted first team coach not an all controlling old school manager. It will cost a lot more but we need to do it.