What this article doesn't mention is why vacuum tubes are still used in guitar amplifiers:
The "distorted guitar" sound common in rock music is a direct result of nonlinear characteristics of tube amps (both the tubes themselves and the coupling transformers necessary to connect an output tube to a speaker). Tubes exhibit "soft clipping" when pushed to their limits, which has the effect of producing the harmonic content associated with distorted electric guitar. Rock and roll pioneers got their sound principally from driving tube amps past their limits. "Overdrive" pedals are usually just voltage boosters that make the amp's input signal louder, but this pushes the amplifier past its clean linear section into the nonlinear crunchy overdrive that guitar players want.
Modern tube guitar amplifiers, even "new" designs, generally follow one a handful of basic circuit topologies, all derived from a few classic designs. Even the iconic Marshall "Plexi" amp was a close copy of an earlier Fender. These circuits are extremely simple; a handful of tubes, transformers, and passive components, often less complicated than the internals of a single LM741. Here's an example:
https://www.thetubestore.com/lib/thetubestore/schematics/Fen...
It's possible to mostly recreate this sound with digital signal processing or carefully-tuned solid-state analog circuitry, but players like the sound of tube amps and are willing to pay for them, and so the market exists as it does.