They already limit signups in oversubscribed areas, and as good as starlink is, it's still a technology that has higher latency, and higher ongoing costs from what I can see.
Meanwhile, once fibre is in the ground, with PON there's very little maintenance required. And upgrading speeds is a case of upgrading the OLT and ONT's - I'm going out on a limb and guessing that's much cheaper to do than launching thousands of upgraded satellites.
Which they are not even close to yet. Starship is about launching the suckers in bulk.
My FTTP ONT is using less than 2.
That's 788kWh per year ((90 * 24 * 365) / 1000) and at 25p per kWh that's an extra £197.10 of energy usage per year. Compared to £4.38 for my FTTP ONT.
It's cool tech, no doubt, but it's not a replacement for a fibre connection by any stretch of the imagination, and so the positing that it'll be the, or one of the, bigget ISP's in the world seems off base, most likely by an order of magnitude or more.