I do think that IF it happened, young drivers paying 2-4 mil for a couple FP1 sessions a year would still happily pay for the 2 young driver designated test days. Okay, there is less coverage and less fans in attendance and worse is that they are through the day testing that isn't conducive to being on tv due to the time slot/length. But ultimately those young drivers are paying 2-4mil upfront for the shot to eventually make 10-20mil a year in wages and be F1 drivers, they'll still pay for that chance.
So I'm not sure the income from young drivers is a particularly genuine concern. It's not like given a test that a sponsor maybe paying for the driver to have the chance will find the team changing the livery with said new sponsor(afaik, if they do I never notice so it's not working
).
The biggest hit is with fans at the track, when you reduce it to one session in the evening a lot less people will bother showing up on a friday. Surely that also hurts the potential income for every other event on during the day.
Get rid of friday totally, which will hurt the sport, or keep both sessions.
You wonder if sponsors would be happy with their names on cars being cut down from 7 hours(roughly) to 5.5hours, some teams might well find sponsors saying, 20% less time on track, we'll pay you 5-10% less(because quali/race obviously have much higher viewing figures). Then you could end up losing more than you save.