The thing that annoys me the most about Newcastle fans is that they seem to have this thought that they should be a top club, but without the history to back that up, they literally have won nothing (aside from old Second Division and Championship) in 51 years, and they royally blew their chance at the title in 1995/96.
They’re not a top club with any history of success, but they’re definitely big in terms of local popularity.
I agree with your point that they’re not or should not be considered a top club, but they are definitely well supported. I now live in Bristol and it’s clear that Bristol City have a huge following. Right now they would sell out a 40k stadium if they had one, so I think there are ‘big’ clubs under the radar that are more deserving of attention than others.
Newcastle will never struggle for support.