You can probably get 100gbit/s between satellites, maybe more. A single fiber link can already support several tbit/s, some submarine cables can transport 40tbi

You can probably get 100gbit/s between satellites, maybe more. A single fiber link can already support several tbit/s, some submarine cables can transport 40tbit/s or more. And the Atlantic and Pacific are full of submarine cables, 20k satellites won't be enough to replace any of that. Just the amounts that Google/FB/AWS etc push between continents is more than a satellite link could sustain. For latency sensitive applications (e.g. finance, video links), starlink can indeed be a good alternative but it won't replace backbones, e.g. distribution of youtube videos to edge locations around the world.