Saying that people who have access to a public squat toilet in a rural area have "access to [indoor] plumbing but don't use it" might also be taken as disingenuous, but I suspect you were mainly not thinking carefully.
No one fails to recognize the convenience of indoor plumbing. Running water right in your home is incredibly convenient. With a properly constructed drain system, it means you can also have a completely private place to defecate, indoors, a near-miracle of convenience.
I do not think that this is a fair characterization of the proposal that is being rejected by some in India.
It is possible that you have been inside a public squat toilet in a densely populated rural area, or for that matter even a poorly maintained public squat toilet in a busy urban shopping area.
If so, it probably takes little effort to imagine yourself seriously considering defecation in a ditch, or an alleyway, as an alternative.
If one had already been doing such all one's life, then there would be no obstacle of unfamiliarity, inexperience, or or public shame to discourage you. I do not see why one would not generally prefer to defecate in a familiar and comfortable manner, in the open air, rather than a small, dim room begrimed with the stench and trace faeces of thousands of previous acts of defecation, performed by strangers.