Most of the difference in sound quality among condenser microphones comes from the capsules. Most sample-to-sample response variation within a given make and model is due to production tolerances among capsules, which are generally about twice as wide as the production tolerances for amplifiers. Most repair issues with condenser microphones are issues with their capsules. Most deterioration due to age or mishandling occurs in the capsules.
The really critical part of any microphone is its transducer element, which has to convert the miniscule kinetic energy of air molecules into variations in an electrical signal. Once that signal is in the electrical domain, the problems aren't entirely routine, but they are more or less widely shared and understood throughout the whole electronics industry, and good solutions exist for them although they're not always cheap. So in some sense, designing and building the purely electronic part of a condenser microphone approaches being "a solved problem." (Never mind that many Chinese microphone manufacturers have "solved" this problem by copying the output circuit of the Schoeps Colette series, using components and manufacturing processes of whatever quality.)
That is not the case with the capsules, however. They have limitations dictated by physics as well as our incomplete human control over behavior at a molecular level, and they can't simply be made so that they do anything and everything that we might want them to do. A certain amount of what is known, however, is carefully guarded. For the sake of their own survival, all the top-tier manufacturers have production secrets that contribute significantly to the sound, the consistency, and the reliability of their capsules.
This is no myth. I laugh whenever I read a blurb saying that someone has recreated a "vintage" microphone from the original manufacturer's specifications or drawings; you can't get nearly enough information that way. Even the present-day owners of a company (cough cough AKG cough cough) may not know how their predecessors made certain capsules sound the way they once did. And I think people are aware of the rivalry that existed for a while between Neumann (Berlin) and Microtech Gefell over which company has the more authentic version of the Neumann M 7 capsule (answer: neither one exactly, although both are quite good).
Any of the top-tier manufacturers have well-designed, well-made amplifiers; they are all familiar with the problems, and can offer very high-quality solutions. But I do not personally believe that even (say) Neumann, Schoeps, DPA, Sanken, Sennheiser or Gefell could make copies of one another's capsules that sounded and performed exactly like the originals. I think sometimes of the woman at Schoeps who built all their three-pattern capsules for forty years, and her daughter-in-law who assembles capsules for them now, in the only room of the factory where no one is allowed to take pictures. You can't package the skills and methods that come from that level of first-hand experience.
Anyway, with condenser microphones the hero of the story is always the capsule, and the amplifier is more or less the sidekick--who also has some powers, and saves the hero on a regular basis but never gets top billing.
--best regards