This is partly true, of course."or the company could issue a board replacement which would more or less be plug n play."
But even small manufacturers can replace boards, components, modules, etc.
Here is an example regarding my amplifier VR-03B (with Purifi 1ET400A).
New Power Supply Ectifier Board PW-2-PRE for Purifi buffer input stage (2 X OPA1612).
In general, there are no problems replacing components.
By the way, instead of a good encapsulated BV EI transformer, I now install a 2x18V toroidal
transformer specially created for this power line. Toroidal - quieter.
I remain a supporter of classic transformer power supply. I also appreciate the timbre sound of musical instruments (tones).
I continue to improve and optimize the amplifier design...
In general, this is not some kind of special business project.