I think it would be a much better strategy for the computer to want to take out major threats. No player is going to sit on small, non monopolizing territories and watch another player gobble up and solidify continents, if they have the ability to block them. That's an intentionally self defeating strategy.
For instance, in my last game it was down to myself, the computer who took over for a player who left the game, and a player who had north and south America, and Africa. The computer took a couple of European countries, and went through a couple of Asian countries butting up against Alaska and stopped. The computer could've easily taken north America and at least broken the south America monopoly and didn't. That makes no sense.