Is there some way to tell that a distinction is doomed to disappear? When did the difference between presently and currently disappear? Presently and momentarily? The distinction between imply and infer seems to be hanging on, but will spell-check inevitably kill the difference between principal and principle? The question is not whether the disappearance of distinctions is good or bad, but when the tipping point -- or tipping range -- occurs.
Garner's Modern American Usage has a useful feature called the Language-Change Index (page xxxv), whose purpose "is to measure how widely-accepted various linguistic innovations have become".
The Language-Change Index has 5 stages, as outlined in this Language Log entry by Mark Liberman. Garner's key to the 5 stages includes 10 amusing, if idiosyncatic, analogies. So the Olfactory Analogy lists the 5 stages as follows:
- Stage 1: Foul
- Stage 2: Malodorous
- Stage 3: Smelly
- Stage 4: Vaguely odorous
- Stage 5: Neutral
And the Etiquette Analogy has:
- Stage 1: Audible farting
- Stage 2: Audible belching
- Stage 3: Overloud talking
- Stage 4: Elbows on the table
- Stage 5: Refined
Garner includes a lengthy discussion of how he decides on the stage that any given usage has reached in his prefatory chapter The Ongoing Struggles of Garlic-Hangers.
As to the confusion of infer/imply, Garner says (page 464):
Writers frequently misuse infer when imply would be the correct word. ... Don't be swayed by apologetic notes in some dictionaries that sanction the use of infer as a substitute for imply, Stylists agree that the important distinction between these words deserves to be maintained.
Garner rates the use of infer to mean imply at stage 3 of the language change index.
He rates presently for now or currently at stage 4. And principle for principal at stage 2.