Please explain how solid state is more prone to failure.
Let's look at it from a parts count point of view. The more parts you have, the less reliable over time it will be. One part which relies on another part, which relies on yet another part has one failure, and no worky... The more of these co-dependent parts there are, the higher the probability of failure. Also, consider that there are now that many more parts that can fail.
Now let's look at the difference in robustness of the different design types. If one amp has 5 transistors, and two or three IC chips, there are now that many more resistors, diodes, and capacitors to integrate all of them. Looking at the more old school Fender design, it has tubes, resistors, and very few if any other supporting electronics to make it work. Tubes are very reliable, and much easier to swap, troubleshoot or replace. Resistors are expendable and the fewer there are, the easier it is to troubleshoot. and then there is the actual troubleshooting of the two amp if one does break. IC's, and transistors ( especially if there are many of them ) are not as easy to troubleshoot and in some cases not easy to replace. There are more of them too, oftentimes.
Solid state amps have more parts and use more run-of-the-mill parts. Heck, many of these newer SS amps use SMT technology. What is wrong with SMT you ask? Nothing....Until it breaks. A real tube amp is less likely to be miniaturized and will be MUCH easier to fix if it does break. They are often more robust just from the design alone. Perhaps using the word " prone " was a little harsh, but truthfully, over time, it will be more prone to fail. There are more parts, often less robust, and if it does make it 20-30 years, will you be able to find the replacement IC or transistor that was used?