Fair points, but I'd say our situation is a little bit different to those clubs.
Brighton is a reasonably sized city, with a population of around 250,000 and a huge catchment area for fans due to there being no other teams in the local area. They have previously had attendances in the 20,000s in the 70s and 80s but the sort of figures you're talking about came when they were in the 3rd tier, in an out of town stadium with no parking facilities, temporary seats, and a running track around the pitch. A lot more than the capacity of the stadium has changed for them, so the increase in gates isn't necessarily indicative of a long term increase in demand. Wigan is a good example to use to see what I mean by that. While in the 3rd tier they moved from an awful, awful stadium to a brand new one with a larger capacity but their average attendances remained under 12,000 for another 7 years before suddenly rocketing to over 20,000 for their first season in the PL. They remained around 18/20,000 for the duration of their time in the PL, but have been back below 12,000 for the past couple of years now that they're out of it. It wasn't the stadium which had increased their attendances, it was the fact they were playing Everton. Liverpool and United instead of Tranmere, Shrewsbury and Macclesfield. Similar stories can be found at Hull and Bolton, who've both 'lost' 7-10,000 fans since being relegated.
West Ham is also different simply because they're in London. There are 9 million people living in London, and a constant stream of millions upon millions of visitors, many of whom are all too happy to take in a game of premier league football while they're over. Fulham used to benefit greatly from those fans, and West Ham now get a lot of them, particularly as a trip to their stadium can be teamed with a visit to the Olympic Park and the country's biggest shopping complex.. I'm not sure that the population of Liverpool and the visitor numbers we would get could support the same easy growth that West Ham saw.
We need to increase our capacity substantially, there's no doubt about that, but I have to admit i'm slightly sceptical that we could improve our attendances by 50% overnight. Maybe if we were winning every week and had great players to watch, but would 60,000 be turning up to watch a Sam Allardyce team lumping aimless balls for Oumar Niasse to chase into the corner? Personally I would have no problem with a 50-55k capacity.
