Its not always on. Its only 'always' on if you would orbit the sun which starlink can't do, it has to orbit the earth. This only works in a certain constelation which would create a halo around our planet, without clear understanding what even would do.
The more power you consume, the more power you need to dissipate. These constelations wouldn't be small at all. It would also take a interesting solution to be able to move this heat from very small very intense areas to very big cooling areas. How?
And space is not easy. Space is very very cold which puts a lot of stress on materials. It has radiation. And it has A LOT of microasteroids. Stuff in Space breaks down due to this. You would need to replace all of this stuff regularly with resources from the planet earth.
You would basically just spend a lot of resources throwing a lot of resources out into space. You can't even recycle all of this.
Its still lunatic at our current state of our current system. There is so so much space on our planet. Its ridicoulous
The only reason Musk is saying stuff like this is because he knows there is no market and he needs to keep his system alive
A polar low earth orbit can be always-on (no earth shadow). Each satellite will be in thermal equilibrium, around 10°C. Catastrophic destruction from micrometeoroids is rare. I'm not saying it's a good idea, but I don't see any dealbreakers in the math/science.
The always on orbit exists and is called a dawn-dusk Sun synchronous orbit. It is an orbit that is always above the terminator (line between night and day) where it can face the Sun 100% of the time.
This orbit has to rotate about a degree every day to follow the terminator as the earth orbits the Sun. It uses the equatorial bulge of the earth to achieve that rotation without have to spend rocket fuel. It is really quite interesting.