I'm for 18-22.
Let's not forget that the enjoyment we receive from F1 is provided by human beings. They deserve time off and time with their families. F1 is very demanding physically (I've been a business "Road Warrior" -- travel can be exhausting). Add to that the stress on each member of every team. Let's not burn them out.
And -- monetary value aside -- too great a supply of anything can reduce its exclusivity and therefore perceived value.
I also think testing has value for:
tracks without an F1 race to prove themselves
young drivers to prove themselves
engineers to see their simulations in the real world
Finally, back to testing and perception: I think it's important to at least suggest to the non-obsessive F1 fans that the cars are designed by fallible humans and therefore NEED to be run to be proven and improved. I WANT the teams to have the time to push the envelope as far as possible. With budget restraints and even more races, development could suffer.