>the casuals that liked to pvp got driven out by the experts and tried to start again in a new game.

I find this to be a really interesting point. Many people would ask why they are playing a PvP game in the first place, if they're not interested in competing. I think it makes a lot of sense though, because they're not actually all that interested in playing against players. They just want more of a human-like challenge than what AI can provide. The only option there is to play against other players.

A friend and I used to keep in touch by playing RTS Vs the AI while chatting over the mike.

The AI in most games is utterly banal and predictible, and as soon as you go past the easy or medium settings it cheats ruthlessly. Of the big recent ones (StarCraft 2, Company of Heroes 2), it's been a very disappointing experience.

For SC2, it just keeps rushing you, so you end up turtling until the AI runs out of resources.

For CoH2, again it just constantly rushes you with much better stuff that you can afford. CoH2 is actually really annoying as you can never afford anything, and so don't get to play with different units, you're always seem to end up playing the same composition.

What this means is that every game is virtually the same, you're just trying to establish a line, wait until the computer runs out of something (or uses the artifical unit cap for stupid things, effectively running out of an army), and then you roll them.

The only game I remember with a little variance was Dawn of War 2.

Maybe co-op Vs AI is something they just don't care about, but it feels as if it's got worse over time, becoming extremely predictable.

This. My personal favorite game for co-op vs the AI was RUSE - they had several different AI profiles that tended towards certain strategies you could specify, or it would choose at random. However, it really did respond to what units you built and what strategies you chose, building bunkers against a rush and such, or building a ton of anti-tank weapons if it saw you had tanks.

This meant that subterfuge and concealing your intentions was legitimately a huge part of the strategy - if you intentionally let the AI see a lot of a certain kind of unit and then build a different kind of strategy, you'd often see success, but because of the way the AI strategised it could sometimes meet you head on with an unexpected surge because it briefly saw you building up from an unspotted recon unit hiding somewhere.

Ultimately it is relatively easy to defeat with a little experience, but I still several years later occasionally boot it up with a friend to try and beat the hard AI 2v4 - we've only managed a handful of times, and the experience of slowly being whittled down back to back is a great experience to have in a friendship

It's not that they don't like to compete. It's that over time that the bar gets raised so high that they can't. People get too good while their skills don't increase as much or remain static. Or the effort needed to remain competitive is too much.