After over 50 years, why has no realistic alternative to valves been found?
Part of the reason is that valve circuits are blindingly simple.
Every alternative approach we have seen involves greater complexity.
The best sounding examples of valve amplification do not employ a technique known as "negative feedback", which is widely used with solid state amps. Consequently subtle variations in the individual components and circuit layout can greatly affect the overall sound. Because of this, it is not uncommon for two identical valve amps to sound and feel slightly different.
Part of the "Black Art" in so-called boutique valve amplifiers is the knowledge to turn these changes to the advantage of the overall tone. Point-to-point wiring is often cited as a key technique to minimise stray capacitances and signal pick-up, which can otherwise lead to mediocre tone.
A good ear on the part of the builder will enable weeding out of undesirable changes introduced by "bad" components. Even changing a valve can make a significant difference in the tone. Sadly they do need to be changed from time-to-time, usually at the most inopportune moment.