My daughter is profoundly mentally and physically disabled, she is 33 but under 3 months developmentally,
As a child, she was completely unresponsive, her brain injury was so profound that she wasn't able to interpret the signals to enable her to learn to see or hear, she just existed.
Being a child of the 60's I did to her what my mum did to me, everytime she had something to eat or drink, I signed toilet on her body (moved her right arm across midline and tapped her left shoulder) whipped the nappy off and strapped her onto a handmade throne,
The poor love, she had no trunkal stability but was hoiked up on a potty. Long story short, perseverance paid off, it took a few years but she got conditioned into 'signing' and using the toilet, it can be done, but sadly, for various reasons the pads had to go back on, but I still feel proud that she managed it.
Her older brother was dry day and night by 2, I just whipped his nappy off and said you're a big boy now and it was as easy as that for us!
When my children were small, the nappies were horrible and I don't think they kept the wet off you like today's nappies do, so maybe now that they are so absorbant, children don't feel uncomfortable being in a wet pad so the urge to get out of them isn't as great?