[NikkiA]

Airline pilots had an engineer at the time before GPS was available, asking a 2-crew system to handle VOR-to-VOR or (non-FMS) IRS navigation as well as their regular navigation load isn't really feasible.

That said, most airliners still have a backup IRS, and can still fly a regular flight plan with IRS and no-GPS, it's just 10 seconds extra setup time at the start of flight.

(trying to go back to IRS-only transatlantic/pacific flights might be a problem due to drift, but GPS still works outside of the small area Russia is jamming anyway, so we're not at that point yet)