Forgive me but I disagree.
There is a broad spectrum between Jefferson (4x5@95%, level 1) to Unicorn +1 (60 minutes @95-100%, level 10). The first might feel easy to moderate while the latter probably will feel very hard to all out. Both workouts have their place as not every session is required to be a productive, stretch or even breakthrough.
Also, picking up your example, I think it would make sense to decrease levels after a 5 watt bump from 4 to 2 or even 1. At least as per my experience: Before my last bump I did polar bear +2 which is rated 12.5. After increasing my FTP, my sweetspot level went down to 7.x and this has proven to be just perfect. I certainly couldn’t get away with a 9 or 10. So yea, I guess for some it might work.
However, the absolute decrease shouldn’t be a reason for concern. Your feedback will directly influence how quickly levels increase, how long they stagnate or perhaps eventually decrease. So, again, as per your example, if you rate a prescribed threshold workout as easy, AT will adapt. Maybe not after the first but most certainly after the second “easy” rating.