As such it's kinda a moot point to attempt to discuss anything about such a study, since it's locked firmly in the realm of hypotheticals and opinions.
Me not having time to count my calories or exercise doesn't mean i can just pretend I won't get fat because "it isn't practical" and then expect it to be so. Lack of funding or knowledge is not an excuse to just go pretend things are going to work out or that you know things anyway.
I absolutely agree that that study isn't realistically going to get run. That does not change the fact that it would need to be run if you want to justify the current US policy. Without it, the government can and should only have a policy of ensuring either [minimum level of vaccination their data do support, which is probably around "60% or higher"] OR [level of vaccination that is known to break endemic disease levels, which for measles is at most around 75-80%], whichever is higher. And to remove penalties for people not vaccinating above that (retain them when the rate is below)
Both are currently less than 90% and even more less than 100%. And either one would be easy to advertise and justify in 1-2 sentences.
Other than that, once you achieve those levels, people should just be left free to follow their own intuition, since it is no better or worse than the science above those levels. Like your own personal intuition to get vaccinated, which is just fine.
it's better politically and medically to simply choose one side of the argument and stick to it until proven different.
Why? This is not how science works. Without positive results, you keep an open mind, not "pick something randomly and promote it..." Go ahead, go attempt to submit an article to any peer reviewed journal and tell them "Well I don't really have data for this, but it would be SO HARD to get any, so I just chose this theory by coin flip. Please publish. Kthxbai" and see how that works out.
I admit that as a scientist I'm also not a politician or a medical doctor, though. Can you further explain a reason why it is better for them to ignore the scientific choice here and "just pick one side" instead?
(Edit: Note that the above, lack-of-data coverage rate is NOT the same thing as an actual known optimal rate, and should be published by the CDC and related to any laws, etc. accordingly as just "we don't know how much higher than this is good or not." not "This is definitely the best.")