Living in the Midlands where there is not one particularly massive team but many local clubs (unless you think Villa or Forest are massive clubs, I don't), I can say that I have seen many more Man City jerseys than ever before. There were none up until 10-15 years ago, as Man City were an irrelevant yo-yo club. Shit, I can hear my next door neighbour screaming when Man City are playing, and that fucker is about as Manc as I am. Being a big Stone Roses fan back in the day doesn't qualify you. A lad a few houses up switches between Man City and Coventry jerseys when I see him about.
So yes, it's true lots of people support their local club but you'll also find in areas where there is not a massive club, or no notable club at all, fans will naturally gravitate towards successful clubs and be "plastic fans". You'll quite often see them at local games or supporting the team but they also randomly pick a big, successful team based on any random parameter. I also noticed over the years that the majority of British Asians will follow a successful club ahead of their local one. It's mostly always Liverpool or Man Utd. And also the children of those who glory followed Liverpool and Man Utd 20-30 years ago follow on from their parents.
When I was growing up in the 90s one of my school mates supported Leeds and another Blackburn, why do you suppose that was lol? Walk around here and it's all City, United, Liverpool, Chelsea and Barcelona jerseys. All good local teams.
One thing I will say is that most don't often switch teams once that is set in stone, but there will still be a few wankers out there.