Considering the level of inconsistency in the Manchester United team under Erik ten Hag, I don't think there's anything wrong with your opinion is absolutely valid. If Manchester City or Liverpool were to be the team that's facing Southampton this weekend, I'm very sure you'll have a different opinion but because it's Manchester United, there's every possibility that they'll be beaten by the Saints.
Inconsistency does not give Ten Hag right or privilege to fail match to match, and if he performs like this, he will lose his job. It's easy, as a boss, assume I am, I care more about results, temporary results and plan for future. But having 2 seasons and this one is a third season, but fail to get acceptable temporary results, is bad enough to forget about any good long term plan. When belief is destroyed, a long term plan will become bad and unbelievable, the contract will be terminated.
Ten Hag must know that risk increases with time and each poor performance, and he must wake up as soon as possible for bringing good results to Manchester United boss. Competing with Manchester City and Liverpool in this season is impossible for Ten Hag and Manchester United because their quality is not good enough.