[Dedas]

Well population won't win you any wars, gold will however. So if you treat your population like any other resource you will see that you must invest, transforming it into castles. Castles means more resources and resources means more troops. With troops you can get more provinces and thus more population to tax.

If you play defensive and are not expanding rapidly I can see that killing population will mean trouble. But that trouble is nothing compared to the offensive players heading your way.

Also, in late game gold isn't needed much (except for upkeep), gems are. To get more gems you want magic sites and that means lots of provinces - see above.