If he continues to fail into the next season then you sack him.
And no, no one can really reset mid season. He needs a transfer window to right some wrongs (and he's not going to be the sole lead on targets this time if he stays so I don't see a risk there), and he needs a revamp on how his medical and support side evaluate injuries and manage players. It's not easy to reset mid season.
But that wasn't really my question mate, if we have a decent season next year 3rd-4th. I presume you would keep him for a 4th season yes?
But then if the wheels come off again in season 4 I'm sure you'd be of the opinion that he'd definitely get sacked, at which point we'll be in the exact same position we are now. 1 good season followed by 1 bad season, but we'll have wasted another 2 years, 4 years all together on the Ten Hag project. That's why I don't trust him to take us forward, the dip this season can't all be explained by injuries.
I could accept a bad season this year and chalk it up to injuries, but it wasn't just bad. It was diabolical in terms of results, team performances (many players regressing), nonsensical tactics etc etc. For me you can't go through an entire season playing the same nonsense tactics that clearly aren't working, tanking the season in the process and keep your job. I think Ten Hag is a good coach, I do. But I reckon he's blown his United career through stubborness, as I still think he'll get sacked soon.
It isn't easy to reset mid-season, but it's not impossible either. And for some reason he didn't seem to think his job was on the line, so he just cracked on. Maybe there was no one in place to say anything to him with the changover of power.