a) It sounds like what you're saying is "you'll get poor people". There are several things wrong with that, the first one being the rampant classism inherent in it.
b) So you'll get horses, and mobile homes. So what? Oooh, are you afraid your property values will drop? Deal with it. That's a small price to pay to enable the kinds of walkable neighborhoods mixed zoning allows, and the kind of rejuvenated communities it creates.
c) Allowing residential and commercial zones to mix has nothing to do with people trying to build their own septic systems.
All in all, it sounds to me like you experienced what happens when you live in a lower-income area, that happens not to have strong zoning laws, and your takeaway from that experience is that the lack of zoning caused the lower-income parts. Correlation is not causation, and not letting rich people stuff poor people away in a corner and forget about them is absolutely part of what we need to do.