Yeh they're loaded, but seeing as new consoles will be out within 2-3 years, and in that time valve might release 2 games on that new engine, after which, the engine they've just made would be outdated and would have to be updated signifigantly - it just makes more sense for them to wait for the next gen of consoles before they work on a new engine as it will have a longer lifespan. I highly doubt a new engine would make more people buy dota2 or cs:go as they're both games that have a hardcore player base that will play regardless or not.