Fair point, although he didn't blame Johnny for everything - I thought he was generally very kind about Johnny in Autobio.
From the outside, it looks like the rot set in when the public backlash started against Moz in 2018 (and the questions started coming to JM about Moz ruining the Smiths legacy, etc). I think the media stirred up the pot and who knows - maybe something happened in private before that, but I think the real 'turning point' was then.
I also think that Morrissey believed the headlines and the press more than what Johnny actually says. It's ironic considering all the headlines he himself has generated and the way the press has treated him.