On accident is not even considered a mistake by most prescriptivists.
It's a generational thing, old people use "by accident", young people use "on accident".
Stop yelling at the proverbial kids to get off your metaphoric linguistic lawn.
I think you're romanticizing "correct grammar".
"Correct grammar" is merely taking a random point in time, picking up a dialect (which happened to be the dialect of rich white people, but that's surely just a coincidence) adding some random rules pulled out people asses and deciding that's "proper English".
There's nothing magical or special about "correct grammar", it's not superior in any measurable way (and there are linguistic ways to measure those things) to any of the many other English dialects.
Also, you're wrong in thinking it's gotten worse, English now is waaaaay more standardized than it has ever been.
Go read some old texts, if "should of" make you angry, that shit is going to give you an aneurysm.