I'm not going to get drawn into the debate over whether they pose a risk to cyclists or not, beyond saying that I've never had a problem. If slipping is a problem, is that not a question regarding the friction of the surface, rather than it's profile?
On a more practical level:
Ok, if the parallel lines won't work because some partially sighted people use wheeled walking aids and presumably (from what you are saying) cannot feel the ground beneath their feet, why not use bobbles?
Because they're not directional and they are used to mark crossing points at 90 degrees to the path's direction (assuming I interpret my observations of their use correctly).
...if a number of people here, who represent a tiny portion of the cycling masse, have found problems with parallel lines, why keep them?
Because it's what people are used to. Changing it would be a little like swapping to driving on the right - do you do it overnight (imagine the logistics of digging up all that pavement), or do you phase it (in which case how do blind people tell which way the path runs in the intervening period?) Whether it was well-planned or not, it ain't going to change. Live with it.
(Edited to sort out the quotes)