I've enjoyed grinding my T3 city. I still enjoy grinding to improve its decorations. Doing these things I see as my personal accomplishments which I've chosen as some of my goals in playing AoC. And, to be honest, I really don't care that I have any other benefits from owning a T3 city. The stat benefits are unimportant to me.
...the point here is that you wouldn't unsub if I took away your city tomorrow provided you still got all the benefits of it.
Actually the opposite. If my city disappeared, I'd probably be so disappointed in Funcom that I'd seriously consider playing something else. If they took away the benefits, I probably wouldn't notice.
But there are people to whom the continuing and neverending grind is a very strong incentive to unsub.
You can tell that by the number of people who aren't playing this game.
You are likely right about this. But - and this is my point here - not everyone thinks this way and your assumption that anyone who doesn't must be wrong (or in need of professional help) is misplaced.