I can plug my Macbook into a 5V only USB phone charger and it says it's charging (ok, it doesn't really keep up with the power usage, so the battery still drains, but it drains slower than it would if not plugged into anything at all)
Also, having both step-up and step down convertors in your laptop is going to make everything a bit heavier, bulkier, and costlier, for the very tiny proportion of users who want to charge from a 5v phone charger overnight.
Check this app note from Intersil/Renesas: https://www.renesas.com/eu/en/doc/whitepapers/power/usb-c-bu...
(of course, it may be slightly less efficient in boost charging, but since the 5V usb source has limited power, this may not be a concern in terms of component thermal limits etc).
It is super annoying that I can't use any of my power banks as laptop-like batteries for Raspberry Pis, Arduinos, and the like.
It's hard to say that the poor implementation of USB-PD ruins the XPS 13, because it's overall a great laptop. But it is very annoying to travel with this computer, realize that you brought only an 18w or 30w charger, and contemplate how nearly every other device you have works fine with it. And every year since 2016 I decline to upgrade my XPS 13 because they haven't fixed this (moreover they probably don't even consider it a problem).