I agree, I really dislike that the metric is "of adults". Just as bad when it's "of eligible". It should just be "of population", no need to do fancy math on what that means. If that means the target is different, that's fine just say what that target is.
A super good reason why the metric should be "of people". When Thursday's new vaccination number comes out, not all of it will advance the goal of "of adults". The whole number wasn't in the past either, but it'll be worse now.
Making it harder to know where we are.
People and the news WILL confuse the two. We'll see lots of comments and news stories on how many were vaccinated and how that gets us closer to the goal, but it'll be over counting the daily contribution.
Which is why everyone using "of population" would have been better.
Better yet would have been metrics based on spread, but we've all determined that's to hard and to abstract. Settling on Vaccinated was always a proxy. As such, it was also always more conservative than it needs to be. It would be so much worse to set a proxy measure that's to aggressive and doesn't actually lead to the spread reduction. Especially as the spread reduction will lag the vaccinated metric.
Let's say we told everyone that 35% was enough (to exaggerate wildly) and then when got there, there were still 500+ people dying everyday. There would be outcry, calls of useless science, how could the number be so far off, we cannot trust anything anymore. It would be bad. So instead, the metric is conservative and errors on the other side. We'll probably get the outcome we want first before we hit it. But, it's only "probably" not "definitely" and nobody giving a target wants that risk. Combine in all the "of adults" and other fudge factors and the target is going to error on the side of even more conservative.
Even in the reporting of the "70% of adults by July 4" target, the assumptions of what that really means are all over the place. Many will be disappointed if getting there doesn't mean "everything is dropped". No matter if that's what was meant or not.