It's not "obviously" a pretext. I don't think that the change in ad revenue
to Google is going to be significant between v2 and v3 ad blockers. It might be to ad networks and sites that employ significant ad blocker counter measures though.
And it's not "altruistic" - it's because eval() and webRequestBlocking are bad for security and performance, so they're bad for a lot of users. Users who will switch to Safari or another browser without that extension API, because the browser is faster or didn't exfiltrate their banking credentials.