> Constellations can do undeniable good for the ground, but I think it is really a choice between dark skies and global satellite internet.
It's a good summary of the situation, yes.
> I don't think SpaceX can lift LSST-class instruments to orbit...
Literally - not soon, but the launch industry is at a point of fast evolution, so it might make sense to use it and device different kinds of astronomy tools - not multibillion gems, one-of-a-kinds, practically irreplaceable in case of launch failure, but numerous, with fast turnaround, sensors enjoying short design loops from conception to using and benefiting from decreasing prices of kilogram on orbit.
Frankly, some LSST-class instruments can only be based in space, and SpaceX can well be a good helper in getting them there.
> or make it possible to photograph the Milky Way rising over a national park without streaks in the sky.
With today's photographic equipment, yes. With naked eyes, unfortunately, yes. But there could be a non-insignificant replacement.