In regards to your first line, it probably is just about true. After his first season he was comfortably more bad than good in each season. I do think part of it was the managers over-playing him so it wasn't all his fault, but still if we're rating his performances we have to call it as it is.
Even his first season was really only 50/50, where it was the most perfect sliding scale of performances over a season that I think I've ever seen a player have. Started brilliantly, dropped to good after a couple of months, dropped to decent around the turn of the year, dropped further after a few more months and was struggling quite a lot by the end.
In 18/19 he started the season utterly dreadfully. The only reason he wasn't the worst player in the team was because Lukaku was showing near record levels of shit-ness. Matic was quite good again for a couple of months after Ole took over, but then declined again and was average at best for the last few months.
19/20 he struggled all season bar about a month on either side of the Covid lockdown where he was good.
20/21 and 21/22 he was mostly poor, with the odd good 60 minutes every couple of weeks but anything more than that and he just couldn't handle it as the opposition ran rings around him.
Like I said, I do think a significant amount of his struggles was that the managers were playing him too much when he couldn't handle it anymore (Mourinho in particular basically played him every single minute of every single game). So I do wish we could have his time over and play him more suitably. If we'd done that I think he'd have been a hell of a lot better than he actually was.