100% of new cars is critical because of how long cars last. People can keep a car running for 50 years after it’s sold, getting rid of ICE cars requires making them unappealing. As gas stations start to close owning an ICE car will become extremely inconvenient.

A hypothetical 10% of new cars being ICE eventually means only ~10% of gas stations survive, that’s just not enough. It’s vastly more efficient to do that in lockstep so people aren’t stuck with relatively new cars their unable to fuel which then get scrapped.