We do have an idea what the next war will be like. Just look at the Russo - Ukrainian war. But you are right about one thing assumptions are the mother of all stuff ups. However evidence so far of Russian GBAD utilisation shows that it is either technically poor or I think more likely, the training and actual war use are two different stories entirely. There is a distinct disconnect and it's not just in that either. I agree that blanket statements like ''Russian GBAD is not a threat to the F-35'' are fanciful at best.I actually did and it's self explanatory : 'what I'm doubting is the notion that on paper capabilities will lead to the expected results in actuality'.
I have no doubt that the technology is impressive and significantly improves the ability to perform SEAD/DEAD. What's good on paper however and what actually occurs in reality can be profoundly different; I will not assume that just because a particular side has a major technological edge; that it's given that in an actual conflict we'll see what we expect or hope to see - in a previous post you mentioned assumptions and basing facts around that.
I will also not make blanket statements like ''Russian GBAD is not a threat to the F-35'' for the simple reason that we have no idea what a next war will be like and that the enemy also has a say. It would be silly to assume that just because the Russian GBAD has perform relatively poorly in the Ukraine that this will be case in a Russia/NATO war or that technological superiority is a guarantee for anything.