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.
Offering something then taking it away is materially different from never having offered it at all.
Abuse of extensions was certainly a problem with IE. But, Google was also happy to use IE controls for Google Toolbar and other functionality. It irks me when a company/tool makes use of another's more permissive policies/APIs in order to gain a foothold and then restricts others from doing the same with its products. It feels like pulling the ladder up behind you.
The reason why Chrome waited for so long to add extensions was the danger they posed to users. I was at Google when Sergey often worried about what extensions would do to non technical and older users who get tricked into installing them, then I saw first hand that danger with my own grandparents. They had extensions intercepting every network request, redirecting certain sites to fake sites, and injecting code into pages. It was horrifying, and they were lucky that they didn't have significant money or identity theft.