I see your point and it is certainly consistent with the traditionalist sensibilities I often read in your posts. It brings to mind a discussion from a linguistics class I took in graduate school about prescriptive vs. descriptive interpretations of language. A prescriptive interpretation says 'this is the way it's properly done'. It's locked in, formal and largely unchanging over time. There's certainly a place for that as an anchor point to better understand the origins of contemporary usage.
Conversely, a descriptive interpretation observes and accepts trends in popular usage, saying 'this is the way people are doing it now'. The reality is that living languages evolve over time and words take on different shades of meaning as our perception of the concepts behind them change. You only have to watch a movie or TV show from the mid-20th century to see how much our language has changed over a relatively short period of time.
Your examples both of vacuum cleaners and of cars illustrate this point nicely. At one time in the not too distant past, there was a apparently a greater distinction in the vacuum cleaner industry between canisters and tanks. These days, so few of either type of machine are being produced that the distinction is lost on the average person and it is thus more expedient and comprehensible to refer to both styles collectively as canisters. Likewise, the distinction between a four-door hardtop and a four-door sedan or a coupe and a two-door sedan is lost on the average person who does not know, or maybe doesn't remember that some cars didn't have B-pillars. These days, thanks largely to modern safety regulations, true coupes and hardtops have been effectively out of production since the late '70s. In today's market, all four-door cars are sedans and the few two-door cars still being produced are coupes. The term 'hard top' today simply means 'not a convertible', although some convertibles today have retractible hard tops instead of fabric or vinyl soft tops.
For better or worse, language and words tend to change with the times and as that natural process happens, earlier meanings--or shades thereof--are diminished or even lost entirely. It's not a new concept; it's been with us since the beginning of, well, language.