Have you given any thought to the cost of dumping “used” air into the extra-building atmosphere — or more accurately, the cost of replacing it with similarly conditioned air?

First, the potential cost of equipping a space with a far UVC bulb
Far-UV-C bulbs are starting to appear as LEDs. If you consider visible-spectrum LEDs The ones that output 2000-ish lumens (like a 150–200 incandescent) draws about 20W — and indications that power requirements for UV (not far-UVC, I am making a leap here) LEDs are similar.
Given that far-UVC doesn’t need to be shielded to protect humans we’re talking something like a medium base (or dual pin) fixture that, even built for a public space, won’t be more than about $500, plus maybe double that for installation.
So 18 KW/year (20Wx24x365) plus fixture/installation costs, the actual bulb ($100?) and replacement every five years (guessing low on bulb life). Probably close enough for cost comparison — but remember this is just for one bulb, a large room will need more

On to exhausting air — and the cost of conditioning the replacement air:
Best case, (e.g. a classroom with a wall of open-able windows) you can just open lots of windows and you have to spend the energy (either heating or cooling) and maybe stick an exhaust fan in one. So, maybe $300 for the fan, 5–10W to run it, and heating/cooling varies with the average degree days in your location. But even in that best case scenario, the heating /cooling system was not designed to heat/cool “the outdoors” — so you likely need to spend $$ on upgrading that. They might also use heat recovery ventilators, which, for residences, start at around $1500. No idea what would be needed for a public space, but I’d guess a minimum 10x that. There’s also the possibility that it isn’t possible, for whatever reason, to upgrade that system (e.g. limited boiler room space, limited duct air capacity, limited steam radiator / baseboard heat exchange capacity).
Somewhat worse case, the room does not have open-able windows, so upgrade ducting and air handling units.
Yet worse again, a small space with many people using it (e.g. an elevator).
I’d hesitate to name a “very worst”, but some universities have underground / subfloor classrooms that already have inadequate ducting just for CO2 — those would be a decent candidate.
And… there isn’t just a pandemic on, there’s an issue with using energy that puts CO2 in the atmosphere. I’d guess that 18 KW/year (20Wx24x365) is at least an order of magnitude less energy than exhausting / reconditioning — and it’s in electricity — so if not already non-CO2 generating, at least amenable to change to that as new sources come on-grid.
I’m a little surprised that this doesn’t just “jump out” to everyone; probably best that I spell it out, particularly for people that live in clement climes.

TL;DR: works fine for places with open-able windows in San Jose or Honolulu; rest of the world, not so much.