Sure, but (a) crew are cheaper than vehicles, and (ii) even if the crew need their breaks then, it still doesn't make sense for doubling of service to require more than doubling of resources. It should be more efficient to keep vehicles in service as much as possible and have crew swap between them instead of having vehicles sitting idle for long periods of time ... they don't need a PNB between journeys!
For example crew need a 30 minute brake they arrive on the xx:00, which then becomes the xx:15, the crew from the previous train (having been on the incoming train 45 minute earlier) run the xx:15 whilst the crew from the xx:00 then run the xx:45.
That allows for a 10 minute delay before there's questions about breaks being too short.
Conversely the xx:00 arrival becomes the xx:35 departure, either you hope that trains are on time all the time so the staff have their 30 minute break or you have to have spare crew anyway to ensure that have at least a 30 minute break (in which case your extra staff costs to go to 2tph from 1tph are definitely not double).
Anyway, if you want to look at where the big costs of the railways are (year to March 2024):
£4.1bn TOC support
£8.3bn NR grant
£9.5bn NR/HS2 investment in enhancement
£0.3bn other
As such, excluding investment, the big railway cost is the NR grant, so if you can do anything to spread that across more passengers the outcome could be a lower per passenger amount of support (even if the TOC level of support increases a little).
If the person tax payer support doubled to £840 but passenger usage was such that everyone made on average 100 trips a year by train (up from 25 now) then far more people would see that as good value.