Reminds me of the discussion with breasts and bras, where now they're saying bras actually promote breast sag because the coopers ligaments, which function to hold the breasts up, don't get to work since the bra does their job, and since these ligaments don't work, they don't strengthen or repair when needed since theres no stimuli. The penis though is different, the shaft doesn't have ligaments like in the breasts, its mostly just smooth muscle.
I do notice though personally that going commando makes it more likely to hang, being contricted in underwear makes it adapt to the limited space.