To be honest, they have no business in the Premier League. London has around 16% of the population of England, and 30% of the Premier League clubs. Adding Watford to that is completely unnecesarry ...
Watford isn't London or even Greater London.
Interesting looking at that map and seeing how many teams there are from the south/south east. That's a big swing compared to a few years ago when the likes of West Brom, Stoke, Sunderland, Cardiff, Swansea, Wigan, Hull were all parts of the Premier league at some point.
Brighton, Brentford, Bournemouth, Fulham added to the PL regulars (Arsenal, Spurs, West Ham, Chelsea, Southampton, Palace) means you have half the PL in the south east or on the south coast.
If you went back to 2011/12 there was 8 clubs from the North West (Liverpool, Everton, Bolton, Blackburn, City, United, Wigan, Stoke).