It is very far from clear that Starlink is even sustainable. It needs many times more full-price subscribers than can afford it. Expect available data rates to plummet as more sign up.
And, anywhere with trees around (e.g., here), it doesn't work.
And, anywhere with trees around (e.g., here), it doesn't work.
On the “financially sustainable” question: Current public reported runrate is $40+mm/month in revenue = $500mm/yr ARR business on roughly 400,000 customers. Customers are still limited by geo for the reasons you mention - they need more satellite density, so I presume more customers would sign up than have right now, e.g. there is a lot of growth ahead in raw revenues. That should give the current business, as is, at least a $5bn valuation, frankly quite a lot more given the ease with which they can sign up whole countries worth of people.
Can starlink add another 400,000 customers for less than $5bn? Absolutely. SpaceX has paid the capex for lift already, and right now it’s a matter of adding satellites and creating antennas and paying (at-cost) lift rates for the cheapest lift known to man. Starlink is going to be a $100bn+ business IMO very quickly, like next 5 years.
Per your point, e.g. ‘is it good enough near trees’, or what percent of the earth can Starlink serve? On the one hand, it doesn’t matter a lot because that number is clearly north of 10%, and that makes the starlink broadband market 3 times the size of the US as is — and that ignores the various super-high-value markets like shipping, jets, and elsewhere that will pay 50x a ‘typical’ customer - but, I think the tree/coverage problem in general is a problem of low satellite density right now.
My starlink is laid out as best I can near my house, and it’s at like “96% coverage” per the app; every ten minutes or so, it loses connectivity for a bit. That’s because all available satellites are occluded by my trees - with additional satellites, I wouldn’t have that problem. So, I would guess that problem gets less bad over time as they grow.
And by my estimate they need a million subscribers just to pay for replacement satellites as the old ones burn up.