There was a time in Chrome when it didn't support extensions at all. If Google had release an extension API like manifest v3 then, would that have been abuse of market position?
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.
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.