Note, first of all, that this is a claim about words and meanings, not about matters of usage.
The claim is, as a matter of simple fact, historically true. It also doesn't have any serious consequences for arguments here over meanings. It's also true that if you use a word to mean something it doesn't yet mean, no one will understand you. And some meanings that develop may be as matters of historical fact, what the words mean now, but they will still be unfortunate developments for other reasons. Take, for instance, the usual shibboleth here: it may be true that it is coming to be the case that with regard to grapes in wine, varietal has the same meaning as variety has with regard to all other life forms, including grapes when wine geeks aren't talking about them. Developing a special word for wine geekery for a biological category, though, and particularly doing so without regard to its taxonomic illogic, seems to me both unnecessary and coming from a toxic blend of ignorance (about terms in biological taxonomy) and elitism (wanting a special vocabulary when none is necessary). My students frequently write doggy-dog world when they mean dog-eat-dog world. History may make doggy-dog as acceptable as under way now is for under weigh. But in each case a metaphor is lost to the language because of the historical development, with no real gain occurring.
Much the same argument would probably apply with regard to grammar and usage. It's probably true that the subjunctive is falling out of the English language (witness the first sentence in the quotation). Grammatical complexity commonly falls out of languages over time. But the languages do become poorer for it. In this case, not only will sentences not distinguish between hypothetical and contrapositive statements, on the one hand, and declarative ones, on the other, but writers may start not to be aware of the distinction.