I'm curious, where else have you lived with such a great balance of everything?
#2 suggestion: Chicago, IL. Museums (check), bars (check), restaurants (also a Michelin city), nature (Lake Michigan).
#3 suggestion: Boston
I will 100% grant that the weather in the Bay Area is generally better than all three of those places - but if you don't highly rank outdoor activities, does that matter as much?
Come now, you aren't suggesting that the National Mall counts as "proximity to nature" ;-)
I've spent some time in all three cities and live in one; none of them really offer the same ease of access to nature as does San Francisco, if you count nature as necessarily including some level of remoteness from the built-up environment.
With density that I until recently used to consider as an unquestionable positive becoming at least temporarily moot with covid (and frankly becoming permanently soured by the out of control protests - I realized I prefer my density Singapore style, with CCTVs and harsh sentencing), I can see why people would move out. I'd probably be out of Seattle, at the very least to the burbs nearby with the same access to everything and few of the downsides, if it was just me making the decision. Same applies to SF (I've lived in SF and Mountain View before, I'd say it's even more acute in SF, quality of life is just terrible even for someone who loves dense cities - might was well move to peninsula or east bay for better access to nature in essentially the same place).
To be fair, I've never been to Rock Creek Park, but photos of it make it look like any number of state- and municipal-level parks near me. :-)
https://www.loc.gov/law/help/statutes-at-large/51st-congress...
If you're willing to travel as much as it takes you to get to Big Sur from San Francisco, you can easily visit Shenandoah or the Blue Ridge, both bona-fide up-to-snuff pre-approved Real™ Beautiful™ National Parks from DC. There's also the Chesapeake, tons of magnificent beaches and state parks in the surrounding area---many beautiful sites on the east coast had already been state parks far before the notion of a national park was invented. That doesn't make them less beautiful or grandiose, however.
And at that point, you lose a lot of what makes SF appealing versus other cities.
As for weather, some people do value not having snow or typical summer heat/humidity (or both). But SF isn't the only place with a nice climate (and, for many, SF is gray and foggy relative to even other nearby California options).
What are other options for similar mild weather? LA/San Diego gets too hot, Atlanta gets humid as well, Boston and NYC get too hot and humid in the summer, and are snowy in winter (though I don't mind that so much). Seattle's got too much rain.
But coastal California (which, as you say, is mostly somewhat different from SF specifically) is the only real Mediterranean climate in the US.
This particularly unpleasant spell of weather we had until a couple of days ago notwithstanding, New England generally isn't bad in the summer especially once you get out of the cities. You're rarely going to be too uncomfortable on the Maine coast in the summer even without AC. But, yes, it's cold and snowy in the winter.