> Is it a cost thing or a capability thing? i.e. is it possible for Starlink to have what they want without interfering with terrestrial astronomy or are we really choosing one or the other? Quite sympathetic to astronomers if it is cost thing, but undecided (maybe slightly leaning Starlink) if it is capability thing.
I think it's a bit of both; right now SpaceX is trying to get the brightness of their starlink sats down to ~7th mag (they're at 5th mag now, they have an experimental coating that should bring this down to ~6th mag), and it sounds like if they can do this the problems will be far more manageable (for LSST specifically this amounts to a few extra months of observing time, however the impact even with these mitigations is strongly dependent on the specific science case).
However, this work should be done before approval. What happens when SpaceX realizes that getting things down to 7th mag is too difficult and costly and time-intensive? Without regulations in place, I would expect they shrug their shoulders, do some PR to frame the problem as "us vs those pesky astronomers" and launch what they want to launch. We have virtually no leverage. But from the standpoint of people that operate in space pretty regularly, what we're asking is pretty basic. Astronomy of course isn't going to be thrilled with the amount of satellites growing by a factor of 10-100x, but we live in the real world. But something that monumental needs to be done thoughtfully and carefully and with respect to the billions of dollars of R&D already paid for that will be impacted.
> Assuming that within 20 years we can have sufficiently cheap space launch systems that there are sufficient geosynchronous telescopes for amateur use would that solve this problem?
Again, space delivery costs are only one factor. Not to say this wouldn't enable new kinds of instruments or make some new things possible, but it would not solve the problem, no. There are too many things that can't realistically be done from space.