It's an odd word, isn't it? I can't speak for the inimitable Walter Bright, but I suspect many English speakers like myself enjoy the pure sound of it - so many consonants in such a short space of time. Somewhat difficult to pronounce, slightly archaic, it perhaps confers a little more kudos than commoner alternatives. (And thoroughly deserved kudos, in this case!)
They also use names that seem bad choices today such as Inconstant, Inflexible and Terrible (that word changed meaning over the centuries. Originally, it meant “terror-inducing”. https://en.wikipedia.org/wiki/HMS_Terrible lists eight ships with that name, going back to 1694, so I guess that stil was the intended meaning for the one from 1944)
Consider terror, terrible and terrify! The second word now generally means something like a rubbish or bad state and the third means to induce a state of the first word (bowel quaking, etc etc).
The word dreadful (almost certainly also a warship name) has not suffered quite such an ignominious decline but "I'm dreadfully sorry" is quite often seen in the wild.
I really enjoy words like that. If I haven't seen it before, or it's been a while since I've seen it used, I need to take a bit of extra time to read and decode it.
It also reminds me of words like 'irreparable', rolls off the tongue quickly.
Now, when I hear someone use the word 'extant', especially during spoken language, I immediately think it's pretentious.
The idea of word use being pretentious is self-absorbed and anti-intellectual. I occasionally use the word "extant" because it's part of my vocabulary and is the best word to express its meaning, not to impress people that I don't even know exist prior to their complaint that using some word that is unfamiliar or uncommon to them is "pretentious".
