If Starlink succeeds, it can easily deliver recurring revenues of >$5B yearly. This will dwarf all the other revenue sources SpaceX will have in the next few decades, and easily justify the current valuation (and even a substantially higher one, for that matter).
The moment Musk made the call on Starlink, he made all the other ambitions of SpaceX conditional on it. If Starlink succeeds, they can fund Mars colonization out of petty cash.
20 million global Internet access subscribers would mean at least $15-$20 billion in revenue (assuming Starlink can't fetch the prices globally that they will be able to in the US access market). Starlink will definitely be a large business if it's successful, although their early projections on potential were outlandish. Comcast is a reasonable target for scaling comparisons.
Comcast access + tv is essentially a US only business. They have something like 26 million access subscribers and 22 million cable tv subscribers. The access business alone is good for $25-$30 billion per year in sales. They've accomplished that scale just in the US market. A bunch of medium income and developed nations still have relatively slow Internet access, including France and Australia. There are an easy ten million Starlink customers waiting just in the giant US market.
Starlink will never fund a Mars colony out of petty cash though. It'll take nearly every dollar of profit the business generates to establish a Mars colony.
You are estimating the average Starlink customer at 1000 USD/year? That seems significantly above average mobile and landline Internet fees, so it would seem hard to acquire 20 million customers while competing with those?
People said the same about Iridium. It's entirely possible that Starlink could work as advertised but not have subscribers to deliver that level of profitability.
