Usefulness. At some point a new port standard will emerge for things other than charging. And will become the new standard. Now, if it remains backwards compatible with the current form factor and pinout of USB-C, then nothing changes. But if it is different, manufacturers will ask the EU to change the requirement.
It may become
_a_ standard, just as USB type A, type B, type B mini, type B micro, type B micro USB3, and type C connectors are all standard.
What's not a given is that any future type D connector would become
dominant for power input given that manufacturers would gain no competitive advantage by implementing it in their mobile devices - in the very best scenario the industry as a whole
might switch over to a new connector
much later than if companies were free to implement it on their own, driven by the interest in gaining a competitive advantage over their rivals.
That's essentially the reason we even have USB Type C on many phones in the first place rather than micro-B - at first some companies implemented it as a feature competitors didn't have, and later many competitors implemented it to catch up with the competition. Remove that ability to choose independently - by forcing most manufacturers to lobby a government entity over several years to effect a change - and you remove both the incentive to implement a newer/better technology and you drastically slow down its potential rate of adoption.