Observe your average older person who lives a life employing furniture pick something up off the floor, or pick themselves off the floor. It's not uncommon for them to need assistance in the latter case!
If they had no furniture all their life, unless they were injured, they wouldn't have these problems because they couldn't possibly go through life without being able to get off the floor regularly having no furniture around.
It's also proven to encourage me to move more throughout the day, as well as stretch more. The floor is spacious, it's luxurious, and unstructured. It invites me to move around, stretch, and reconfigure myself when one position gets stale or something is going numb.
Furniture is structured, it's almost like bondage. You must sit in this chair design this way, your back must be up at this angle, your legs go here. It's totally unnatural and honestly seems like an awful compromise in part to increase density, and perhaps project wealth over those who live in furniture-less, largely empty rooms, or maybe it's just good business to sell furniture to everyone. And in the process it's preventing you from having to bring yourself to the floor and back up to standing height dozens of times every day.
There were some studies I read about geographic regions with the largest concentrations of the oldest living people. I don't have the link handy, but one of the things they observed was an exceptional number of these places didn't use furniture. They noted how the lack of furniture naturally kept the people limber into old age.
I love sitting on the floor now. It's become incredibly uncomfortable and simply annoying for me to sit on furniture. When I'm with my peers, and I'm not even that old - if we all end up sitting on the floor somewhere, I'm shocked at how visible the difference is already. Many adults are not able to be comfortable on the floor becuase they don't have the necessary flexibility for the variety of floor positions that would otherwise be available to them. It's because they stopped spending time on the floor after childhood, because in the west successful adults use furniture. You want to be successful, don't you?
I'm not a good person to ask about time to adapt, I stopped sleeping in beds as a teen opting for the floor. So I was already pretty well acclimated by the time I decided to get rid of the rest.