You are right — they are definitely misusing worse and worst in those examples.
However, I think the impression that this error is catching on may be an illusion — the usage isn't really changing, it's just such a jarring mistake that when you meet it, it really stands out. Googling the phrases you mention, compared with their correct versions, gives:
"That's the worst thing I've ever seen." 51,200 hits
"That's the worse thing I've ever seen." 598 hits
"This can't get any worse" 1,320,000 hits
"This can't get any worst" 9,580 hits
Google hits are an awfully rough measure, but these differences — factors of around a hundred in each case — show pretty conclusively that this isn't a common usage. If anything, for words that are so close together in both pronunciation and spelling, I'm surprised there aren't more people making this mistake as a typo or thinko.
The phrase "I can't hardly wait" is incorrect.
I suspect it is the result of a confusion between:
I can't wait
and
I can hardly wait
which are both correct.
The phrase
I can't hardly wait
doesn't make sense: it would mean "I don't find it hard to wait", which is probably not what is meant.
Probably adding to the confusion is the 1998 teen movie
"Can't Hardly Wait". It is possible that the title itself was picked up because the expression is in vogue in American high schools although I did not find any confirmation for this hypothesis.
Best Answer
It's not really a question of opinion. Stupider is, as the dictionaries and usage you quote show, entirely grammatical. It is also in rather common use, though less common than more stupid.
The main issue here is that words of more than one syllable tend to resist the -er suffix. Wiktionary's entry on the -er suffix says this (emphasis mine):
I believe that people's aversion to this word stems from the belief that "-er is usually used for monosyllabic words" is a rule, rather than a guideline (or, more correctly, an observation). This tendency probably has phonological roots related to stressed/unstressed final syllables interacting with the suffix, but I really don't know enough to say.
If a word has a meaning everyone can agree on, appears in dictionaries and is in use for centuries, this means it is a word!. It might be a word with social connotations of "improper speech" or "bad grammar", but a word nonetheless.