They still had by far the highest percentage of vote - almost as much as the next two parties combined, so it's still clearly a mandate. I agree that we should move from FPTP to PR, but having said that, I disagree with your general attitude toward why - a couple of reasons.
1. That attitude is of a party seeking power, not one in power. In power seats count, Labour has them, they need to use them. I don't remember the vote count causing the Tories paroxysms of hand-wringing both-sidesism. Act like they have their own ideology and aren't just empty vote-chasing suits.
2. It's obvious to everyone not stuck inside the tiny Labour echo chamber that pandering to fascism is exactly the wrong approach. The knife edge is of their own creation. They need to start acting how their voting base expected them to, not internalise the policies of the far right.
3. Maybe Labour MPs need to get off X. They're sat drinking at the Nazi bar listening to the chatter of Nazis. Look at what it's doing to them.
In short, "let's do what the US Democrats did" will end in the same tragedy. No doubt they'll only discover Keir isn't viable shortly before the next election and put another candidate up at the last second. But then, like the US Dems, it's increasingly apparent Labour are just empty suits.
I should also apologise when I said "your attitude", it's how you perceive Labour's approach, not your own. Sorry about that. Obviously my quite apparent rage is directed at Labour, not you!
I agree with much you said although I do think they act like a party seeking to retain power at the next GE rather than being in power now & using it in the national interest. They've lost their direction & are being blown around reacting to polls & the right wing agenda pushed in the media.