Currently Microsoft keeps FTDI in this monopolistic position and without it it is dead in the water. FTDI's competitive advantage is it relationship with Microsoft, not its technology or pricing.
Yeah, it's nothing to do with decades of writing and maintaining drivers that actually worked, while other manufacturers came, went, screwed up, disabled clones, and generally were a pain in the arse.
There is no FTDI monopoly, anyone is entirely welcome and able to register their own VID/PID, supply drivers and silicon, and off they go. There's inertia, but I don't think you could sanely call it a monopoly. Not least because Prolific exist, and have suffered exactly the same problems that FTDI are addressing.
You're aware that FTDI have a wide range of variously spiffy products, all with legit drivers? There have been alternatives to FTDI for years, but nobody's bothered, since the FTDI gear pretty much works. My gripe with FTDI, such as it is, is that they let other manufacturers hang on their VID/PID coat-tails for so long, we all took it for granted. (And bricking the devices, rather than some other way of forcing a USB-enumeration Bong-Bong-Bong noise and an error message, was a dick move).