From experience I see that USB-C charger that was included with Pixel charges my SE 2021 via Lightning just as fast as original Apple charger, so why would they change anything (I mean the charging speed when switch over to USB-C)?
The speculation is that Apple will only allow their chargers or chargers they approve (and get paid for it) to charge future iPhones faster. This is the EU telling them not to do it. USB-PD is a standard (already used by Apple on laptops) and can do up to 240W. No need for proprietary chargers.
Why would Apple do this? Money.