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)

Commercial air traffic density is much higher than it used to be a couple of decades ago. It's no longer just a question of navigating, but also of staying in your alloted volume of space during the flight.

GPS is relatively new (only the last 30 years or so).

There was a whole set of navigational aids available to pilots beforehand; NDBs, VORs, TACAN, LORAN, RADAR, etc, etc, etc.

AND OF COURSE, THE BEST OF ALL, THE HUMAN EYE AND A WAC CHART.