It's not meant to replace terrestrial networks, it's a space-based alternative that serves areas carriers have no financial incentive to cover. Terrestrial cellular towers cost between $150k to $500k per tower, and are not economically feasible in less populated areas. There are also many dead-zones in mountainous regions, since cell signals are blocked by mountains.

Starlink Mobile supplements this, it's simply cheaper for mobile providers to partner with them than do their own buildout. Currently only 5% of the earth's surface is covered by cellular signals. Starlink will push that up to 85+%, and is backward compatible with existing cellphones.

5G Non-Terrestrial Networks (NTN) is already part of the 5G standard. It's not a replacement for terrestrial carriers, it's an expansion that enables devices to be always connected and select the appropriate terrestrial vs satellite connection transparently. ~75% of the land mass on Earth has no cell coverage, ~90% if you include the oceans. It's the same transition in theory that we had from landlines to cell towers.

It doesn't not seem like anything approaching a lucrative business.

TAM: How big is the market for high speed internet that can pay $1200+/year and isn't already well-served by comcast/at&t/etc? And of course, this is all with finite spectrum too. So you can't serve the major cities.

No doubt there exist buyers. But rural Montana doesn't have that many households. Add that 5 year replacement cycle and Musk's Trump alignment that has Europe building their own for security reasons.