Meanwhile, C++ lost its full stack status, the only mainstream OS that still offers C++ GUI tooling, is Windows, and none of MFC, ATL or UWP/WinUI APIs are "Boosty".
What keeps Qt relevant as the surviving C++ GUI framework not stuck in 90's tooling idioms are the paying customers, and those have quite clear ideas which compilers and ISO C++ versions they want to have Qt on.
Meanwhile std::string still doesn't provide everything that compiler specific frameworks have been doing since mid-90's.
The problem with Qt not using std::string is when you (gasp!) want to integrate with non-Qt libraries that do use std::string. It has nothing to do with what std::string offers or does not offer compared to any other string implementation. std::string was the string implementation for C++ since before Qt began, and by not using it for their strings, they delivered a significant message about their desire to integrate easily with non-Qt libraries.
But I looked at using Qt in 1999 and easily decided against it, in part for this reason. Those of us coming from a *nix background got the UTF-8 thing much earlier than people affected by the Microsoft worldview.
Thankfully having a *nix background helped Sun getting Java design regarding UTF just right.
Over the years, we've had a lot of pain, of course... But then again, a lot of fun, too, and without Qt we wouldn't have been able to create an application like Krita. Every alternative just is worse, much worse.