The reason to stay in Labour isn't to spite the Labour right but because what happens within Labour is important and affects the country. This remains true no matter how bad you think Labour is https://twitter.com/HealthUntoDeath/status/1276178170442592262

Assuming we're not faced with Tory rule forever, and that another party won't come out of nowhere to replace Labour (both unlikely imo), what happens to Labour is important because how the country is run under the next non-Tory government is important

Hating the Labour Party is a perfectly valid reason to leave it, but it's not an argument for why it's pointless for socialists to stay in it

Just as with struggles in wider society, there's are struggles inside the Labour party. The left doesn't always win, but that doesn't mean it's pointless to try. Look at how the American left has bounced back from Bernie's defeat to win major down ballot primaries in the US

Getting yelled at, my final word on this is as follows: I'm not judging anyone for leaving Labour. That's not what I'm saying. I'm saying you have to fight for things to change, defeat isn't predetermined, and labour is one of many terrains of struggle that matter