Can I just check that your position is that you don't think there should be any line at all? Your position is that an MP should be able to stand election for a party and then quit that party as soon as they won and that would be fine? No moral obligation to re-run the election?
Not dipped my toe into this conversation, but FWIW, yes, that is my position. I can see it would piss the party off, but the candidate is not there to represent the party. Maybe a moral obligation to the party, but not to the electorate. At least until that candidate were to start going against their own personal campaign promises.
In the event that following the party would mean breaking a candidate's campaign promises, then staying in parliament to vote against the party would be the principled thing to do.
We do not have a party-based electoral system. We elect individuals. And those individuals have to represent their entire consituency. Who, given our electoral system, will mainly be people who didn't vote for him/her.
Any change to try to introduce more party-centric conventions or restrictions on MPs without corresponding electoral reform is liable to make the political situation even less pleasant.
About the only thing going for the FPTP system which gives parties totally unfair majorities is the fact that the "party" MPs are in fact totally independent of their party, and have the power to rebel. Take that away while leaving FPTP, and you're going to be in a much worse place.
I can see that for many this somehow doesn't "feel" fair, but there is a rational argument in favour of the system.