Used to be you could just include an RS-232C port, with its mind-numbingly simple hardware and software, and you had connectivity to 99% of the world's devices.
I'm sorry... ARE YOU OUT OF YOUR MIND???!!!
Are you talking about the same +/-12V requiring "rs232" with a multitude of ambiguous, poorly defined, large and expensive connectors that used cables with a randomish number of conductors and signals that were NEVER "right", requiring the average IT person to have a drawer full of "gender changers", "null modems", DE9-to-DB25 adapters, test boxes that allowed you to see and jumper signals as needed, and additional random "improved" higher-density connectors that varied from vendor-to-vendor, that tortured the very souls of many an IT professional for the decades before USB came out?
The connector that Certain Major PC manufacturers decided to re-used for a parallel printer port that needed different signals and cables?
The specification defined specifically for a MODEM and a TERMINAL or mainframe, that made about as much sense as charging for CPU time in the days of personal computers with peripherals like mice and printers and such? The one spec'ed at 19200bps max speed (over short distances, of course.) With such poor data reliability that there were at least three different common schemes for providing flow control (one of which completely threw away portions of the "specification"?
Sure, a UART is conceptually simple. A Serial card was a relatively inexpensive add-on for the original IBM-PC; only 12 chips or so. And if you got one of those newer high-speed modems and wanted to run that new-fangled "windows" OS, they'd ship you a NEW card with a deeper-FIFO UART? (For free, to stop customer complaints that the modems didn't work.) (and custom software, because DOS and/or the PC BIOS had crap support for UARTs.)
Nowadays even very cheap microcontrollers come with built-in UART hardware. But that UART will talk to a USB/Serial adapter just as easily as it will talk to rs232 drivers and connectors, probably at lower cost, and certainly at less stress for both the PC user and the device user (and the support personnel.)
USB has become a bit cluttered with feature-itis, and maybe some cronyism. But in its original form at only 11Mbps and with the "huge" connectors, it was a pretty brilliant solution to real problems that were plaguing the industry.