Well, the error turned out to be that merely adding a RH platoon bat wasn't enough for this offense. I certainly didn't think that was an error at the time though.
But if they don't tender him, that means they are admitting they no longer believe he can play every day. For sure his illness was a factor. There was little indication he was ever going to round into form against RHP even before that, but two months was also never going to be enough time for him to really show that either.
I think when you non-tender someone they rarely come back, they go get their cheaper deal elsewhere, but either way it's hard to see where he fits for the Phillies in anything other than a scenario where Marsh remains the LF (which would still be at Wilson's expense). Or if they do still think the 2021-2023 Hays is in there, in which case the tender is not unreasonable (but still not very exciting to contemplate).
For sure the deal was not a failure in terms of what the Phillies gave up for him.