I ride pretty much every day (commuting, training and racing,or participating as I like to call it).
I am a mechanical engineer when I’m not on my bikes.
I do virtually all my own maintenance. I think just about everyone should do their own maintenance. I used to hold bike maintenance courses for friends and newbies. They too found out how easy it was to do maintenance. Not all of them do their own maintenance, but they all know a lot more than when they started and are able to at least talk to the bike store mechanic a lot more intelligently about their bikes.
bike maintenance is required when you ride your bike. How much maintenance is required is determined by how much you ride. The conditions you ride in, are the other major factor. If you ride in a wet, salt spray enviroment all winter, you will need to clean and lube your bike, plus probably replace drive train components more often than say an Arizona rider.
From my experience, I replace my chain on my commuter bike about every 6 months. I replace the cassette every year. That’s about 12,000km or about 8,000 miles. My race bike gets slightly better attention, but since it gets ridden less, the chain etc gets replace less often (in years, but more often in miles)
IF someone is taking their bike to a store to get tuned up, then it should get tuned up to the best possible with whatever stuff is on the bike. If stuff needs replacing, at least according to the store, then the mechanic should make the call to see if they are allowed to replace a given part.
Some time ago, probably the last time I took my bike to a store for a tune up, they were of the opinion that my front deraillieur needed replacing, so they replaced it. I found this hard to believe as I never actually used the front deraillieur on that bike! I had a suitable sized small chainring, that for my commuting, I never needed the big ring. Firstly, they changed it without notifying me. Secondly they swapped out an Ultegra deraillieur for a freakin Sora one! Why? That’s all they had! Then when I complained and said I wanted the old one back on (as I never used it) they said it was already gone. I said I at least wanted an Ultegra one back on, they wanted to charge me to install the Sora one, charge me to take it back off and then charge me retail for an Ultegra one and pay to have that put on. FK that I said. I took the Sora one off, in the store and took my bike away without a deraillieur. I then bought a new Ultegra one, put it on and ultimately transferred it to my latest bike, where it also virtually never gets used.
The bike store should have called first, and they should have in the case of the original poster as well. Just because the bike store doesn’t think it’s a lot of money, many of us do think parts are expensive enough to warrant careful consideration and careful purchasing.
FWIW I now do arguably all my own wrenching and bike stores are just a source of parts when I need them. Generally I get my parts online, as the LBS’s here seem to take a week to deliver, and I don’t think I’ve ever had a call when my parts do actually arrive, even tho every store *claims *they will call. I hate supporting my local bike stores, because without at doubt, they provide crappy service! I think they only stay in business because they sell pink trikes with white tires and tassles to the kids of the affluent around here.