The "1 million dollar threshold" came out as a backpedal after the runtime fee backlash. Unity's fee chnage was initially pitched as retroactive to all unity games, a per device install, and per reinstall for those devices. It did create a way to harrass and "install bomb" devs with insane fees, along with gouging smaller devs for money in perpetuity that could cost them more in fees long term then they made in profits because it was such a stupid structure.

The fact that they have backpedaled to a 1mil threshold and a "come on guys, this barely affects any of you" is purely because people did react to a very ugly, very tone deaf, and very damaging fee scheme that did affect everyone.