Starlink is profitable. It is not unsustainable at all when you own the rockets and the satellite manufacturing. Read the audited financials if you don’t believe it. Other parts of SpaceX are a different story.
It's profitable if you don't pay for the rockets. The audited financials say the launch cost is $0 for Starlink (and -$whatever for Space [the launch business] but as depreciation over five years and not as direct costs).
So it's almost certainly the case that Starlink itself would not be profitable if it had to pay for its own launches. If Starlink paid for its own launches, the launch business would be closer to profitability but still not profitable, i.e. you'd have two unprofitable businesses instead of one.
It's profitable if you pay for the rockets. Starlink is a massive business now and it keeps getting larger, and it has good margins. It can trivially pay for the rocket business over time.
... if it pays for the launches, it has negative margins. I'm not sure what's hard to understand here.
It might be the case that once you get enough satellites up and replace them consistently enough that margins go positive, and then you can start paying back the cost of "the rocket business" (i.e. more than the marginal cost of launch) but... it's not at all clear that's even theoretically possible. At least not without launch costs coming down significantly from where they are today, which Starship is not clearly en route to achieving.
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