It has been popular in my country. When Denmark won the world championship in 1992, and 20 years from there till obviously 2012, football was the undisputed top 1 sport to watch. We haven't won it since then (and haven't even been close, let's be honest), so that might be why it's just dying here.
What is causing it to go down except for the countries you mentioned I don't know. Maybe it lacks innovation?
France and Italy is stagnant atm, it's only still growing in Spain, the UK, Germany and I guess Portugal too, kinda forgot them. Personally I can't watch it without falling asleep, but I guess it's because I'd rather play it myself rather than watch it. Not to mention the commentators mostly are boring as fuck to listen to. 75% of them doesn't sound hyped at all. I ofc do not fall asleep at stadiums (even tried to go, yes) and I wasn't hyped by that either. Why, I don't know.
I don't mind professional football players, they are physically training hard to be where they are, but I just can't I guess.