My wife recently learnt to cycle (at the age of 48) on a BSO.
She loves her BSO because of that.
She still struggles with hills and the gears cause a bit of consternation at this early stage. No matter, she can now cycle without falling off. I am not aware of when she will need more from a bike than the BSO currently provides. I will be more likely to work that out before she does, maybe not. This was a Gumtree purchase.
My daughter's bike which is older than here (she's 9) is possibly a BSO as well, but she also loves here bike. It fits her, goes well and does what's required of it. It cost me £25 off gumtree, it's a Raleigh Shugo from some time last century. Between that and the wife's bike, I think I'm in to a total of about £50 at this point.
Yes, I have my new road bike, but with C2W, I could hardly say no. I'm just as attached to my gifted Diamondback which, being a gift, cost the square root of diddly squat. Not sure if this qualifies as a BSO these days, care even less.
A BSO is for life, not just for Christmas. As long as you feed it and oil it as required.