Speakers can be upgraded with different parts. Amplifiers, CD players, phono stages, turntables, tone arms- even speaker cables, power cords and interconnects can be upgraded in the aftermarket. Only phono cartridges cannot.
Re-tippers can buy only what’s available from
Namiki or
Ogura (or maybe some others), while the cartridge manufacturers can order exclusive parts patented/manufactured for their own cartridges only. But the question is not even a quality of parts, but a calculation of mass, resonances etc that only original designed can do manufacturing/voicing his own high-end cartridge. Sometime this is a long process and they try different parts before choosing what they are using. Why do you think re-tipper in his garage can do better if he don’t even have an access to those exclusive parts manufactured only for the specific cartridge designers?
You can upgrade weak parts in the products (like amps whatever), not sure how you can upgrade a reference class, you can only change something pretending for upgrade, but only certain kind of people have this habit, most of them are very strange, the best of them design their own products instead of upgrading others.
Cartridge is the most important part in analog chain.
Everyboby knows JICO, their stylus SAS (bury, boron, sapphire) replacements for old MM cartridges is an upgrade for mediocre cartridges, but not an upgrade for top of the line reference class cartridges. Many original styli are much better than best SAS replacement. With MM you can immediately change your stylus and compare, with MC people are waiting for their cartridges from a re-tipper for a 1-5 month and they can’t compare re-tipped and the original, but i think it’s very important to compare side by side the original and re-tipped sample (I did that with some very rare re-tipped/refurbished MM).
Another weird habit is to invest over $1k to a $300 cartridge, some people love it. Denon 103 is one of them, instead of buying a nice $1500 cartridge they are happy to invest $1000-1500 into a $300 cartridge and raving about the result they are hearing.