I work on plenty of "older technology" utilzed well before I was born. I can understand if it was complicated, but all it takes is a couple of minutes to troubleshoot a problem like that. If a "mechanic" has a modicum of mechanical insight that problem would be quite evident and easy to resolve. I have never worked on new disc brakes, but I can guarantee that barring something extraordinary I could diagnose and most likely fix the problem given proper tools and parts. Did the "mechanic" even bother to remove the part? That is what I see happening regularly in many fields; if a problem cannot be determined with the most minimal amount of time (often just a cursory effort) then it is not worth the time to actually take due diligence to do one's job.
Example: I brought my old 3 speed roadster (original except tires; excellent condition, and properly maintained) to a shop after riding through some sandy terrain and needing the chain cleaned while I was away from home. The chain is an original 55 year old component in excellent condition without any signs of wear, rust, "stretch", etc. I merely asked for the chain to be removed, properly cleaned in a solvent bath (not a "cartridge" cleaner), reinstalled, and lubed (that job takes me about 15 minutes, using 8oz of solvent/petrol). I stated that I was away from home and there was no rush, and I could wait a week or more as I had my road bike with me, and cost was not an issue. The first response was: here is a new chain. I said that there is nothing wrong with chain, it just needs cleaning and I am not buying a new chain to replace a good chain (plus, I have plenty of spare chains already). Second response was: here is a chain cleaner (crappy "cartridge" cleaner). I said why would I spend more on a cleaner than the cost of a new chain; neither of which I want or need. Third response: well I cannot get to it today, it might take a couple days. I said forget it; I already told you there is no rush and you could get it done at your convenience. Just straight up lazy and not wanting to the requested and required work. This was not a youngster (mid '30s early '40 year old guy), and he regularly rebuilt and sold used bikes (he sold no new bikes). He worked alone and was the shop owner. As a kicker, I had been there before and purchased some old parts and a used bike from him. He knew me as I was one of his first customers; I never haggled on prices. He was closed in less than 3 years and his online reviews were negative.
Next shop, younger tech; same situation while away from home. I need the freewheel removed (very common Sun Tour removal tool, I can remove it without the tool; job time 30 minutes), and a spacer removed and ground down (I saw a bench grinder in the shop) due to wrong size spacer (plastic, not metal so quick and easy fix) installed when I got the bike. Response: I do not know how to do that. I explained how to remove the FW, although not necessary and how to remove the the 2 cogs to access the spacer. Response: I will need "Jake" to do it. I asked where "Jake" was. Response: I don't know. I asked when he would be back. Response: he's not here today (could have said that at the start). I said will he be in tomorrow. Response: I don't know. So to sum it up the "mechanic" having proper tools and specific instruction on how to do the work could not or would not do it; and had no idea as to what hours their coworker kept, nor if he could do the work. I said forget it (see the pattern), and eventually did the work myself in less than 30 minutes leaving the cluster on the hub. Out of curiosity I asked how much it was to service front and rear hubs, and BB assy; excluding parts (disassemble, clean, repack, adjust); for me 1 hour work at most. Shop cost: each hub $60 and BB $90 ($210 total); no parts. So not only do they not want to or cannot do the work, but they also want to charge hospital rates for basic work. That shop exists in a a large chain sporting goods store (REI) that was known for quality products marketed under their own name at reasonable cost, but is now just selling brand name goods at inflated prices. Sorry for my venting, but cycle owners should expect better. Believe me I have many more stories all occurring in the past 10 years. Note: I did find one shop (in the past year no less!) that did my hubs and BB (early '80s, all loose bearing assy's) for $20 per hub and $30 for the BB; total time for one mechanic was about 45 minutes. That shop manned by a couple 30 yr olds never tried to sell me something, nor balked at doing the work. They also took time to explain problems that might extend work time, type of parts and cost that might need replacing. I knew all of that, but I was heartened to hear actual knowledge being displayed as well as professional courtesy. That shop not only is still in business, but opened up another shop in the same small city that already has about 6 or 7 shops.
I still do not think that the mechanic having to only needing to work on one shift lever was competent in their action and basic mechanical knowedge; it's not rocket science, and IT IS THEIR JOB (I would be embarrassed if I was that mechanic and could not fix it). As with my previous experiences, I would merely have said "forget it", and moved on to a capable shop that was willing to make and honest and competent effort. If that shop could not figure out a simple 80s shifter then how could I trust it to work on more complex problems be they "old school" or contemporary issues.