I agree that Labour won’t go for reform if they win, they’ll get a taste and want the power. And that will be a huge mistake. Our current system does not reflect the country and leads to short sighted policy decisions and no long term thinking. It’s a game to maintain power, the status quo and a lobbyists dream.There is no way on the planet that labour will go for a move away from FPTP system as it all but removes the chance of them getting a majority in the future. As an asie FPTP has served the UK well and personally I think it should be maintained. Proportional Representation facilitated the rise of the Nazi party, if they had had FPTP things would likely have been very different.
I think the reason you mention is often a reason given to trash PR and it is partly born out of shit history teaching in the 80s as well as it being an easy argument for FPTP supporters to make. The Nazis got the largest vote share in the early 1930s and under FPRP would’ve had outright control I think. So many more factors were at play in that situation and since WW2 PR has served Germany quite well.