This interests me. As many of you know, we designed and built our own retirement home, to be comfortable for 2 of us day-to-day, and have space for visitors.
Upstairs we have our bedroom, with an en-suite, and one bathroom for the other 3 bedrooms (honestly, this felt like luxury!). But we have become aware of the issues you describe. When family stay, it really isn't an issue - there is a loo (and shower) in the ground floor and they are young enough to run up and down the stairs easily.
When a couple of our own age stay, usually they don't have to share the bathroom with anyone else, so other than wearing night clothes, it's not an issue.
However we are about to have a couple of our own age stay with us, and I was asked if some other folk could visit overnight and I said 'no' because I worried about the 'sharing'.
As lots of us grew up thinking one bathroom was a luxury, and for most it is still standard, I wondered if I was feeling a bit 'precious' but it seems it is shared by others.
The friends and family that we mostly stay with seem to have a bathroom quite handy that isn't shared with the hosts (as I type this, I feel rather 'posh'!)