Developing niche controllers for individuals and individual conditions is unlikely to be a viable business model, but if controllers are designed to allow some degree of customisation, including the ability to replace one or more inputs with custom external switches (a wide range of on/off input devices, from large buttons to infra-red blink detectors, that all use the same standard mini-jack connection), players are free to develop their own unique solutions to best meet their needs.
This is currently only possible through bespoke modding, which is expensive and unavailable to many gamers. If more flexibility was introduced at a manufacturer level, the cost and difficulty would be drastically reduced.
An Xbox 360 controller with a bespoke modification to allow accessibility switches to be connected in place of inaccessible triggers
Modification-friendly Razer Panthera controller