I know everyone is freaked out by this, but I don't think it's nearly as practical as this is making it sound. In fact, it may just be an attempt by those scientists to get venture capital and bail with it.

If you were going to keep a billboard above a city, it needs to be in geostationary orbit. That is a distance of 36 thousand kilometers. Even if a pixel is 350 sq.ft. and is reflecting a lot of light, it's just going to look like a star at best. They would need an absolute shitload of them to make something that is readable as text or a logo.

If they put them in a low orbit like the Starlink satellites, that's a mere 550 km. At that distance, yes, they would probably be visible. But they'd be moving fast. You'd just see the ad zoom across the sky and would only be visible to inhabited regions very briefly. This'd only even be worth attempting at the handful of latitudes that go close to multiple major cities. But more importantly, you can't deploy a light sail at that altitude. Like, the way a light sail works is it's extremely, extremely thin and needs to be moving through completely empty space, so that the pressure from light exceeds drag even from hydrogen atoms. At 550 km, the molecules of Earth's atmosphere are going to be producing a shitload more drag than such a sail is designed for. Losing orbital velocity (and with it, altitude) is one of the limitations on a satellite's lifespan, and a cubesat can't carry enough reaction mass to fight the drag for very long. The whole constellation would need to be replaced at such an extremely high frequency that I doubt anyone would find it worth it.

At least, it wouldn't be worth it economically. Even at $65 million a launch, Musk and Bezos could afford to burn that every week for the next 250 years, just to put their dick and balls in the sky or something.