Everyone’s has different cadence abilities, the more time training those abilities both high and low can only really be an advantage in my opionion.
As a singlespeed MTBer racing long endurance events (100mile.+) I can quite happily spend upwards of an hour or longer at 45-55rpm on climbs.
In saying that the 40rpm you mention is lower than I’ve needed my range to go for long periods, however, if your body can handle it and you feel good then why not!