The speed of light time to the satellite and back is too long for them to replace courtsiders.

How expensive would it be to rent a hot air balloon and use a telephoto lens?

In the USA, you'd almost certainly be in violation of a TFR (Temporary Flight Restriction) which are usually established over stadiums during games. That is a federal problem that'll get your pilot in a huge amount of trouble.

Did not know that, thanks.

Back to satellites, then.

The Goodyear blimp looks to be getting some kind of allowance to fly over the same areas. I wonder if that flight path has to receive approval from the company that runs the sporting event, or if it's more of a government shall grant situation.

Quite likely the TFR is actually to protect the blimp! TFRs pop up for all sorts of reasons - rocket launches, presidential motorcades, volcanoes, sporting events... They're situational and specific rather than blanket restrictions.

Why not drones?

Low-tech is the most reliable tech. But sure, if balloons won't work due to legal reasons, let's try drones.

Yep, US Open has a roof. Game, set, and match! : )

terahertz imaging, or perhaps multispectral imaging from cell frequencies, 2.4GHz and 5.8GHz wifi?

that stuff goes through walls. and if we can get a THz emitter in the range of affordability, that'd be a neat way to get realtime 3d scans of environments.