1. The only reason there are 15,000 satellites in space is because SpaceX launched about 9,500 of them (Starlink is 65% of all satellites) on their semi-reusable Falcon 9. If fully-reusable Starship pans out, they will be launching satellites at 10x the rate of Falcon 9 at the very least.
2. You don't need to upgrade the satellites, you just launch new ones. The reason data center companies upgrade their servers is because they can't just build a new data center to hold the new chips. But satellites in space are a sunk cost, so just keep using the existing satellites while also launching new ones.
3. Falling solar panel costs decreases the power costs for both earth-based and space-based, but they're more efficient in space so the benefit would be proportionally greater there.
As I said, I'm skeptical too, but let's be skeptical for good reasons.