Tubes in a Valve Oriented Audio System are not a singular entity; they are, as a layman's description, mycelium-like in how there is a network functioning.
The user of the Audio System pretty much has their finger on the pulse and is in full control of the permutations used for Valves across devices. The selected Valves put into service are the systems, Achilles Heel or the Keystone to a Structure.
The Glass Bottle is all but a Glass Bottle that is able to be exchanged for other Glass Bottles by an individual with little experience in electronic matters.
A big responsibility falls onto the shoulders of the individual/individuals who are designing the permutations for inserted Valves.
It is a big ask to think that a Permutation inherited and lived with or designed through selecting options for a valve type is going to hit a home run with immediate effect. Multiple years of created experiences of different permutations are the most likely way to move on with a Valve selection and have the knowledge of what is not wanted to be lived with.
Valves are user-friendly as an electronic component; not much education is required to replace one.
Valves are not so user-friendly or attractive as a cost when creating an electronic circuit that is self-produced, resulting from simple exchanges of valves; plenty can be done by the inexperienced to put the lipstick on the pig.