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.