For me, it's not really about "believing" whether Petey will bounce back. The longer this level of play goes on, it becomes increasingly likely he won't ever bounce back. His injuries could be chronic, which would make a full recovery impossible. And you could probably place me in the most fervent "pro-Petey" camp.
It's about the probabilities of where this team is going to go. If you trade Petey, the return will not include any premium assets. It will be a platter of B-tier pieces like the BUF end of the Eichel trade. Going that route ensures the team will not even be a fringe contender in the medium-term. The likeliest outcome is just a gradual drift toward even more mediocre finishes, even if Quinn stays (I doubt he will). 0% chance of winning anything significant or going on a playoff run.
I don't know what the likelihood of Petey bouncing back to like a top ~15 center form is. Maybe it's 10% at this point. But that's the only semi-realistic route to any kind of contention. They need a top center to contend, and Petey's old form returning is pretty much the only way, as the organization is currently charted, (i.e. not tanking for McKenna), that they will be able to find one. 10%>0%.