i didn't read the original post, so i don't know the context within which this statement was written, but i'm going to call bs. there are varying degrees of "stretched". i bought a road bike with a completely worn 9 spd ultegra drive train in 2007. the guy i bought it from was getting rid of it for a song as he'd been told by a bike shop employee that the dt was worn to $hit and needed to be replaced to the tune of over $350. in that employee's defence, the teeth were getting curvy and the chain had more than 1% of stretch. i inspected the bike and figured it had at least another year in it - swapped the dt onto my commuter (i log about 40 km/week) and have been riding it year-round for going on 3 years - not butter smooth by any means but still going strong. my point is you can ride a "stretched" drive train for years - it's up to you when the thing needs to be replaced: based on its performance, not what some shop monkey says. (again, didn't read the post, not calling anyone a monkey here)
I should have left his post as it was. I also should have said "measured your DRIVETRAIN" instead of "measured your chain."
Gtse was having skipping problems in the back, and all I was saying is that if the mech checked the drivetrain and it was worn (they have cassette checkers that work fairly well too), it's not unreasonable for him to suggest a new drivetrain first, and though that didn't fix the problem gtse went in with, it DID fix a problem he had.
Pretty hard to diagnose one kind of skipping when there's another kind of potential skipping going on there.
I am kind of surprised that you would say this because you are usually fairly knowledgeable. I have one of those fancy Park Tool chain stretch measures and I never use it. I think such measurements are only useful if want to replace the chain before the cassette wears. If you replace both together you can go way way way past what those stretch measurements would give you as rules of thumb. And you can go even further if you are using friction shifters.
My own rule of thumb is when it becomes slow to downshift and I can't dail it in even with new cables its time for a new cassette and chain.
Chain stretch is just one part of the equation for sure, but I've seen many a finished drivetrain that still shifted just fine. It's not usually the ramps that get worn down, it's the load bearing teeth. If I'm assessing for a customer, I'll check the chain with a checker, check the cassette with a checker (especially the higher gears), assess the cassette and the chainrings visually as well, and then take it for a spin and see if I can make it skip; pretty sure most mechanics do roughly the same thing, especially on more expensive drivetrains.
I ride drivetrains till they skip (I don't drink the "replace chains often" koolaid, though I might if I had a super high end cassette), but again, it wasn't unreasonable for Matt's shop to suggest a drivetrain.