Civilian aerospace does not have the security focus that the military has. Even if you solve the security problem from airplane black-box controller all the way to the remote control cockpit, and even if you find civilian pilots willing to fly planes when a remote pilot can override them at any time... there are there costs of satellite communications, costs of designing and installing black-box remote controls on dozens, even hundreds of types of commercial aircraft, and costs of staffing remote control cockpits with pilots who can fly every type of commercial plane, 24/7.

You could crash a plane into the ground and nobody on board could do a thing about it.

You could crash every plane into the ground at once.

Maybe that's preventable by more automated safety systems that override if you're going to crash, but there are other equally bad options and I doubt we could block them all. Flying planes out over the ocean until fuel runs out, for instance.

Writing UAS software is hard. Only operating UAS in warzones helps with the overheads for this. Effectively "drone-ifying" a commercial airliner is a complete nightmare which would probably require government assistance to implement in the form of cutting of red tape or funding the handling of existing red tape.