Actually the better idea would be called a "Mirror AI"...already in CnC3 and RA3...the AI does learn if you beat it the first go, the next mission on the same map...it will tromp you. (However, this is not persistent, because if you change maps or factions the AI resets, and the process restarts.)
However, a Mirror AI is better. It doesn't learn it copies, so lets say you use a particular tactic in game. Well during the game...the Combat AI is fighting...but a silent Mirror AI is watching your moves in Observer mode. The Mirror AI copies all movements, build que, and etc. And then next match subsitutes this build que setting you use as one of a multitude of new tactics for the Combat AI to use. So either the Combat AI will go counter, or it will mirror your earlier attack. This would allow the AI to be competitive and adaptive, and make it a more frustrating opponent. But not all AI's are perfect, if you play any game...there is a sweet spot...usually associated to the time played, which will create a fatal flaw in the AI's coding and allows you to go on the attack. If you play any RTS it takes alot of patience to find this sweet spot.
An example of a sweet spot is fighting for over 3 hours an Extreme AI on Company of Heroes, and the AI has almost total and complete map control. If you hold out, the AI begins to choke, and a gap opens up a narrow one at that...which you can exploit and then take back the map. Star Craft and Star Craft II...the supposedly "unbeatable" Extreme AI's probably have the same sweet spot. In RA3 the sweet spot is the 15minute mark...who ever is on the defense at the 15minute mark will survive and then crush his attacker. I did this in one game where my opponent in a 1v1 game had nearly 75% of all the land and had attacked me repeatedly and several times over had me up against the rocks. But the game lasted into 15minutes...and turned the tide...slowly but surely cutting away his power and forces. The match ended at 34 minutes...with a kill/death ratio of 2.95 for me. And I only had 24 units on the map.
Although, I personally would love to have a CnC game that gives you bonus awards for high kill/death ratios.