A basis of 100 means that the overall difficulty won't change much for big servers (at the beginning less, later more than 100 players), but for small servers it will get significantly easier. And that's good. I saw some level 4 landmarks at the end of era 5 on the scandinavian server.
A basis of 70 would make it harder for everyone. Whether the real average number of players per city really is 100 doesn't matter.