Thanks to guitar players, lots of tubes are being manufactured new these days for old amps. There may even be new tube amps being made.
There is still (!) plenty of old stock of radio and television tubes out there, however.
There are. I had the resident guitar expert at work pick me out a cheap-but-decent guitar+amp combo for my son, and the amp ended up being a 1-watt tube amp
Reckon the demand wouldn't warrant tooling up (and they're messy complex tubes to make).
I find that it's impossible for the average listener to identify the difference in recorded music, especially if there's a lot of distortion in play, but I can entertain the idea that it's a detectable difference.
As someone who moved from solid-state and digital amps to tube amps I find there to be a huge difference in terms of dynamics and response, which massively influences the way I and others play. I can't imagine Jeff Beck or Gary Moore getting the same results with non-tube amps, since their playing relies so much on dynamics and physically feeling and moving air in the room.
edit: for live touring, though, it's so much more efficient to use digital or solid-state options for size, weight and reliability, if it doesn't directly impact the artist's ability to perform
Yeah, I would expect modern software able to do the job for all but extreme cases. Basically, custom Amp mods.
Dimebag Darrell and Buckethead are two names who come to mind, favouring the even response of solid state over the more-dynamic valve amps. If we include pre-amplification such as guitar pedals, solid state is everywhere - a DS1 or a Rat gives that even response for big gnarly cold distortions. Then there's the Boss HM pedal which spawned a whole genre of it's own when players used it as a pre-amp for that unique "chainsaw" sound.
So, you're right that most players opt for valves at the amplification stage, but solid state is all over guitar music.