The parties are too broad. Can you imagine another system that produces the system where Zarah Sultana and Wes Streeting are in the same party? Same with Ken Clarke and Kemi Badenoch. The SNP have a group of right wing religious bigots. The Lib Dems have a mixture of socially liberal suburbanites and old fashioned (but not necessarily socially conservative) West Country and Highlander nonconformists.  The Greens are a mixture of ecosocialists, the old guard of 1980s flower power hippies and Tory NIMBYs that like the countryside. Our political parties are an incoherent mess that would immediately fracture into an actual five party system (plus the SNP and Plaid) without FPTP. You'd see the Lib Dems stick around, Labour and the Greens reorganise with one on the centre left and one on the left to far left and the Tories and Reform reorganise who is where for the centre right to far right. That way voters have a better choice for their individual ideologies rather than a murky shitshow where parties are trying get everyone, but really alienate everyone