I have no problem with 50s and 100s. I do 100k DIY AAAs regularly (not 50s). They are useful in the winter, I find them as difficult/easy as a flat 200k so what's the problem?
Another thing that should be factored in, is that depending where you live these rides can be difficult to plot. I spent a lot of time plotting and testing my DIYs before they actually worked in terms of climbing rate, distance, rideable winter roads, avoiding bridleways, refining the number of controls to a manageable amount etc. In fact, my 100s ended up being either 120k or 140k in order to make them meet the AAA targets and climb rates.
I couldn't do a 50k AAA from my house even if I wanted to because the hills aren't big enough, so my 100s tend to concentrate the hill climbing in sections I ride out to. I suspect that to do a 50k, many people would have to drive/ride to the start which makes me think that they are either of limited appeal, or the ride to the start means they are effectively longer rides anyway.
Finally, if 100s were stopped, I'm pretty sure a number of people would lose interest in AAA and possibly leave Audax UK.
SO LEAVE IT AS IT IS!!!!