Except that once it's made public, other browser vendors can put in their own mitigations as well. Firefox would probably have a patch of some sort within hours.
You might respond that Google should just tell the other browser vendors then. But why just the other browser vendors? Why would Google get to choose who qualifies as worth notifying?
I stand by my assertion. When the public is at risk, and they have the ability to take steps to reduce that risk, it is their right to know about that risk so they can take those steps if they desire.
If there were a gang of thieves working through an area taking advantage of the local population because the majority of locks sold in the area had a major flaw, I would expect it to be reported. That may spur the thieves to change how they operate, and may in some ways make them much harder to catch, but it's irresponsible to not notify the people in danger so they have a chance to protect themselves and their property, and explain what the problem is. They need to explain how it works so people can feel assured any fixes put in place actually fix the problem,and the easiest way to tell if you are in danger is often to try the exploit of the flaw yourself, because sometimes it affects more than the discoverer and the company involved thought.
You either believe the responsible thing to do is to report the details, or never report them. If you believe the responsible thing to do is to report the details, then it's just a matter of when. If it's being actively exploited, like it is here, and if you agree that a year is too long, and no time is too short, then it's a matter of choosing an amount of time that provides the vendor time to fix the exploit while reducing the number of people that might suffer in the meantime. You can either try to make decisions on a case by case basis, which will ultimately be arbitrary, or you can set a policy and follow it.
Microsoft had a week to get a fix out. If it was important enough to them, they would have. They have the talent, time, money and existing resources to do so. It wouldn't necessarily have been easy, but not doing so is a failure on their part, not Googles. I won't fault Google for giving me the choice to protect myself when Microsoft won't.