I see what you're saying but there has to be a critical mass to be able to support a successful team and there are no other towns as far as I can see with a population in the 40k region with a league club, let alone premier league.
Burnley have a population double that of Merthyr and are an outlier in terms of size to be a premier league club, so even if merthyr suddenly doubled their population they would still also need to be a massive outlier in order to be that successful.