I don't necessarily disagree, but he's still had more "bad" games here than "good," and whatever qualifiers one wants to use for why that has been the case, the bottom line is that we're not even scratching the surface of what his total body of work here is probably going to be.
The fact that he's been trending in the right direction, while great for the team in the short term and at least encouraging in the long term, ultimately means nothing at this point with regards to the big picture. He's going to have plenty of ups and downs, both, moving forward, and his ultimate middle ground is still very much up in the air. He's probably going to age out of his prime while he's here. He might end up having to anchor the right side (either short or long term) if things go sideways with Parayko, a role would likely affect his play and how we judge his time here. He'll probably be affected by injuries at some point, the timing, frequency, and severity of which could be anything.
And that's all before we even get into the adding the context of how the team ultimately performs and how his presence/performance influenced that (in his on-ice execution of the role he was tasked with, how his strengths and weaknesses impacted the roles others were tasked with and the team's tactics, etc., and how his financial imprint affected the team's financial decisions, and the subsequent impact of that, etc.).
The conversation goes way beyond "well his signing isn't a mistake because he's playing great now!" And when he inevitably hits a slump, the same should be said for "well his signing was a mistake because he's playing terribly now!"