Don't most of the satellites in the current constellation not have the laser links needed for intraconstellation communication? So they'd need to replace those too to keep consistent coverage.
Nope, all v1.5 sats have them, so everything launched since January 2021. They experimented with them earlier for polar orbit sats. At this point that's by far the majority of the constellation and either approaching or at critical mass for the mesh. They're probably testing it globally now.
To sibling: they definitely need Starship, but not so much for pure launch numbers as the ability to launch v2.0 at all. F9 fairing isn't big enough. 2.0 will let them shrink cell sizes and improve density and performance which they need too (obviously now more in light of yesterday's announcement). More 1.5 sats wouldn't help as much with that, though the mesh will help relieve ground station chokes as well as ilenabling beyond station service.
Inter satellite links are basically fiber, just without the fiber (instead they just point the laser in the right direction and rely on the fact that space is empty). Up and down has to use valuable radio spectrum, handle interference and atmospheric effects, etc.