I am not sure you were as wrong as you think - see definitions 2 and 3 here
Usage of words shifts and sometimes expands over time.
I would personally definitely interpret “apparently” and “plainly” differently - “apparently” to me is “the evidence so far does seem to point this way, but I am not necessarily convinced, or have strong feelings either way” vs “plainly” is “the evidence is clear, I am convinced, and so should you be” - although obviously context would matter as well and could alter this interpretation.
Edit: even your example usage “I’ve been trying to get myself out of that habit, but even judging from my comment history, it’s apparently pretty hard” - to me the usage of “apparently” here indicates similar tension and/or contradiction, in this case between belief/intent (I am trying to stop the habit) and evidence (but my comment history shows otherwise) - it wouldn’t work quite as well with “plainly”
It would work with “evidently” but carry more of a connotation of confirmation and shift the emphasis (I am trying to, but its hard as confirmed by evidence) rather than contradiction (I would like to think I am doing it, but evidence shows otherwise) - of course you might have meant it either way (or even neither) - I am just saying how it reads to me.
Isn’t this how language works ? If majority of the time people use the word in certain way, than that becomes one of its accepted meanings. In fact dictionaries list one of the meanings of “jealous” to be “envious” (with citations of this usage going back to 14th century, including works by Oscar Wilde and Mark Twain that are over 100 years old)