I’ll say it again - vote Labour in and shift the voting system away from FPTP to some sort of PR system - and you’ll likely never see a Tory Government again - you’ll certainly not see as blatant a lurch to the right from a party in power as this one is currently experiencing.
I don't think a Labour government would do that though. For a couple of reasons.
Labour actually do very well out of FPTP. They tend to get more seats than their vote share would proportionally give them. The 2019 result was an outlier in this (and even then, not by much). You only need to look at the Blair years for this.
1997 - Labour got a lower share of the vote than the Tories in 2019, but got loads more seats. It was a similar story in 2001 and 2005. The fact is that if Labour can make themselves popular, FPTP really, really works for them. Why would they change that if they have made themselves popular enough to get a majority?
Secondly, under current circumstances, it is easy to say that a PR system which leads to a minority govt would freeze the Tories out. And yes, it probably would now.
However, things change, parties change, people change.
From a Lib Dem point of view, it is entirely possible that there could be the next Nick Clegg and David Cameron around the corner. It doesn't always follow that the Lib Dems/Liberals would support Labour. On the few times when they did (1929 and 1977) the arrangements lasted no time at all. In 2010 they went all in with the Tories and it lasted 5 years.
Also, there could just as easily be another UKIP around the corner, setting themselves up around some populist cause, taking votes away from Labour but ultimately more likely to side with the Tories.
That doesn't mean that changing the voting system is a bad thing by any means. I might well even vote for it if there is a referendum down the line, but these are reasons why Labour probably wouldn't be falling over themselves to introduce one, unless they have to. And why Sir Keir has already ruled it out.