Why would a serial converter chip need anything beyond “full speed”?
We were not just talking about serial converters as I reckon. The need for higher speeds in a lot of projects is obvious, and this is where FTDI chips give rather straightforward solutions, drivers included.
Granted that for just "serial converters" at low speeds, you can find many chips out there, a lot of them being much cheaper than FTDI's parts and work fine.
You're not going to get 30-40MBytes/s with a CP21xx and a virtual serial port driver. You will with a FT232H/FT2232H (and some others) in synchronous mode and the vendor's drivers in D2XX mode.
You *can* get that kind of throughput with some MCUs which have embedded high-speed USB cores, but it's not going to be as straightforward most of the time.