The real meaning of decimation

Momentarily has meant “in a moment” since the late 19th century. Sometimes (more commonly a couple hundred years ago) the word also means “at the current moment.”

“Oblige” and “obligate” still have different sets of connotations, but I really don’t think your case is very strong here, since their main overlapping senses, in which they’re roughly interchangeable, have been in common use for several centuries, perhaps since the words first arose. The word “oblige” is still extremely common, in my experience more common than “obligate” for this use.

“Silly” originally meant “pitiful” (in the modern sense; “pitiful” originally meant “full of pity for others”). From there, it transformed into meaning “unsophisticated” and then “feeble-minded” or “foolish.” The OED doesn’t have it ever meaning anything close to “young and innocent.”

“Nice” has had a very complex set of evolved meanings. It began meaning “foolish” or “ignorant”, but in the past 500 years has meant at various points “lascivious,” “ostentatious,” “elegant,” “fussy,” “refined,” “respectable,” “virtuous,” “appropriate,” “effeminate,” “lazy,” “fragile,” “modest,” “shy,” “intricate,” “subtle,” “precise,” “slim,” “trivial,” “dextrous,” “doubtful,” “tasty,” “pleasant,” “attractive,” “friendly,” &c.

> used to be that people knew the word "pedantry"

First, “people” do know the word “pedantry”. Second, you realize that the “now” that we use “pedanticism” is since about 1850, right? “Pedantry” is still vastly more common. Compare: http://www.google.com/search?q=%2Bpedantry http://www.google.com/search?q=%2Bpedanticism

> "literally" [is] just used for emphasis.

Not in any kind of careful writing, unless for ironic effect. People still know what “literally” means.

> will the written distinction [between “there,” “they’re” and “their,”] eventually vanish?

Not any time soon?