First of all, stop equating digital goods with physical goods. If you sell me your bike you cannot use the same bike anymore without my permission. If you sell me your software that you own, I don’t know if you are still using the software or not. You can have god knows how many copies stored and I would be none the wiser and frankly not my business either. If you can have the bicycle somehow duplicated, I wouldn’t care either. I just want the bicycle, you can use your duplicated bicycle.

And no, the compensation is not the disc. The disc is just a medium of transfer. You can get the data from anywhere bit-by-bit identical and it wouldn’t matter. The developer gets compensated by the sale of the license. It’s just that in the old days it was more convenient to distribute the data along with the license bundled. Hence the sale of disc is equated with the sale of the license. It doesn’t have to be that way.

Then your point about license keys… That is what is being sold as I stated before. Your license keys is a way to state that you have the right to use the software. If you are somehow planning to sell those license keys, you are technically selling your right to use the software too. Otherwise the software license is meaningless. One person can purchase the keys and sell them for cheaper and cheaper until we get free license keys and freeware. But since people can’t be trusted to play nice by the one who made the software, they implement an online DRM.