If the pay toilets are secure and self-cleaning, and it can be done in a way to minimize water and energy usage, it's not a bad ideal. Some pay toilets have motion sensors, and time limits, so people couldn't camp out in there or do less savory things. If the revenue generated by the pay toilets could be used to support their water usage and upkeep, it may be a better alternative than public urination. Although, sometimes people need a restroom and don't have money, and they should not have to pee in public either.
Using the toilet is something that nobody can avoid. Voiding is a part of human nature, something that we all must do. To have to pay for the use of a toilet is absurd. We cannot always control when our body decides it is time to go, and it's nice to have a toilet handy when needed.
Public pay toilets should not be reinstated because using the bathroom is a human need. Making someone pay to use the restroom is forcing someone to pay for something they have no control over which is inhumane. This is unfair to people with very little money because then they must budget for an additional cost if they were to need to use the restroom away from home or work.
Pay toilets would just result in people who are out of change relieving themselves someplace that they should not be. It could result in a truly disgusting epidemic of public urination and defecation that could result in a health hazard. People should just be able to use the bathroom when necessary.