On the other hand, where would he have gone? He was out of options. He burned his bridge at Ferrari too, Renault was useless and Red Bull had Vettel and Ricciardo. Mercedes was set too, and that right there is the end of teams who could actually afford him.
The only place left that could afford him may have been Torro Rosso, but you can believe he wasn't going to step down to that.
So, in a way, McLaren kind of rescued him and gave him three more year in F1, not that it was productive or anything. McLaren also put together both of Alonso's Indy 500 deals, so without McLaren, the Toyota gig is about all he would have had.
Not excusing McLaren for sucking, but they were about the last team that could actually pay him.
But, you're right that Alonso could go nuclear any minute and throw everyone under the bus, but then what would he have? I guess he could sign on with Toyota who hasn't even committed to racing in the new WEC, or race for pennies in IMSA (where we would love to have him), or bite his tongue and help McLaren pull it's 2020 Indycar program together. Outside of that, anyone who wants him just doesn't have the money.
Or, maybe suck it up with the rest of the team and make it work, which is what a professional driver would do. Of all the drivers who missed the show, Alonso is the only one not saying "We will be back better next year." You're probably right it's as good as done.