well -- it
is a bit of a strawman

but this is usually about the time that I like to point out that RCA introduced the direct-heated 2A3 power triode for audio amplifier use in 1933 per Eric Barbour*. That was 92 years ago. As far as I can tell, the 2A3 has been in continuous production, somewhere, by somebody, ever since. It's a pretty future proof piece of hardware.

They last a good, long time, too when treated properly.
In all seriousness, the
weirdness of the above is hard to overstate. Many tubes have enjoyed long lifetimes because they are good for something -- vacuum tube musical instrument amplifiers, Soviet (or Russian) avionics...
something. The 2A3 has a pretty limited bag of tricks. For "high powered" radio AF amplifiers, it was rather rapidly supplanted by much more practical alternatives like -
ahem - the 6L6 (pentodes, push-pull, and feedback FTW, as they say).
The only "use" for 2A3 tubes seems to be low powered, barely practical "hifi" audio amplifiers -- a decidedly niche market**. Yet the 2A3 is a survivor.
____________________
*
https://www.worldradiohistory.com/A...m-Tube-Valley/Vacuum-Tube-Valley-Issue-12.pdf
** I have one such amplifier. OK, technically,
two. Actually
three, in that "one" of my single-ended 2A3 amps in the form of
monoblocks. 
Dollar for dollar, SE 2A3 amplifiers are hard to beat when it comes to actually listening to music through them.

two US made 2A3 from the 1940s that I bought, used, for $20 each about 24 years ago.

two Chinese-made "meshplate"

(kinda, sorta) 2A3 that came with this amp when I bought it ca. 20 years ago
DSC_0858(2) by
Mark Hardy, on Flickr
two nice, nearly new Slovak-made 2A3 kindly passed along to me recently by a hifi fellow traveler.
All six of the above 2A3 power triodes are alive, well, and ready to
boogie at a moment's notice.

The JJs have been installed for a couple of months now, though -- not in a big hurry to swap in any of the others.