A lot of NFL/MLB teams in the US are currently dealing with building too much capacity in their stadiums during the 90s, when pro sports attendance likely peaked here. One of the challenges is that when tickets become too available in the secondary market, the people who suffer the most are season ticket holders, who suddenly have to eat the cost of a ticket that goes unused (or sell it under face value hoping to recoup something) because it's not as easy to get rid of. If that happens often enough, their season tickets become less attractive and they stop renewing, which makes the problem that much worse (i.e. there's even more tickets available now).
Logistically speaking, the PL is definitely different. First off, relegation is possible so there are far fewer 'meaningless' games in general. Likewise, even in good years, fans in the US often stop attending late season games because the upcoming playoff games become more important, both for the team competitively and for the fans in terms of interest. That's not an issue in the PL.
Still, the possibility of overbuilding is definitely real. I have no doubt that in a 60-65k stadium, they'll have no problem selling every seat for the biggest games, but imagine being a frustrated STH who has to eat the cost of terrible matchup multiple times a year.
As someone else in this thread pointed out, every seat they add is by definition the cheapest seat in the stadium, and it can only get purchased by a fan who is even more a casual supporter than everyone else in the stadium.
I'm not an expert on the matter by any means, but based on the numbers they published (31k season ticket holders, 17k waiting list), 53k doesn't seem that crazy.