I always find this baffling - even Arsenal, Chelsea and Man City have league cup matches / early stage European matches that they don’t sell out. Only Liverpool and United consistently sell out.The fear of empty seats seems to bother a lot of fans more than I believe it should.
You don’t build a stadium for the lowest possible crowd, you want it to maximise revenues 70-80% of the time.
United, City, Arsenal, Chelsea, Liverpool, Leeds, Spurs, and Newcastle we would comfortably sell out a 55k ground. Aston Villa, Everton and West Ham all big clubs that would draw large crowds to the SoL.
That’s 11 of our 19 PL home games - it’s a no brainer to extend the ground.