Are you guessing a million active cities total for all servers in the US, or perhaps worldwide? Because I just went and looked at Arendyll, the first US server, and it only lists 60,483 players on the server. And players 57,314 to 60,483 all have a score of zero. Last time I looked through all my neighbors, I found about 25% active when I did this many months ago, and some of them have quit since then. 25% of 60k is only 15k active cities. Even if you are generous and up it to 35% active, you are still only looking at about 21k active cities.
A couple of other server numbers that I can check without making a new city. Sinya Arda lists 40,704 players and Felydral lists 49,242 players. Just looking at these numbers makes me think we are all way overestimating the number of US players that this game has. Even with 8 servers now, and considering all the players who only play on one server, there is couldn't be more than 150-200k unique player accounts on the US servers. And even if we use an average of 30% across the servers, maybe 50-60k active players in the US.
And on a side note, I think this removing the inactive and try to move the active closer together will have the problem of a lot of the players having the city to not be allowed to be moved. I think a lot of us are going to end up in a wasteland shortly after this is all put into effect.