Reading a few more of his posts I see your point - he admits that he had it set up wrong but still finds time to take a swipe at the "crap engineering" that "somebody clueless has been tinkering with". Because clearly he's qualified to judge something he hadn't bothered to research, while the manufacturers and professional mechanics don't know what they"re doing.

"Personality disorders are mental health conditions characterized by rigid, long-lasting, and disruptive patterns of thinking, feeling, and behaving that deviate from cultural norms, causing significant life impairment."

You can't reason with these people, or help them - it's always someone else's fault.

OK first, the above speaks very poorly of the service shop. We have a problem in the 'States of car mechanics joyriding in customer's high-performance cars, not a short test drive, but long and at full power; This has largely been stopped with said cars having onboard video recording and data recording, and the dealers having to buy back the car at full price. But getting back to you... a test ride on pavement, fine. Returned with mud, the mechanic was joyriding your very expensive bike. About that...

You are running a ridiculously expensive drivetrain, but clearly not a racer. If that's what you want, fine, but yes, cassettes, chain, and chainrings are going to be absolute top dollar. Perfectly functional and still quality drivetrains are available for a small fraction of the cost of what you are currently running. It's the reason I've stayed with 8 and 9 speed rears for so long, parts are dirt cheap, but now even higher-cog cassettes are inexpensive. This makes replacing cassette, chain, and chainring simultaneously a whole lot less expensive. Usually, chainrings are a lot more durable, you can go several cassettes as least with aluminum chainrings. (My steel chainrings are even more durable.)

So I'd reevaluate that service shop, reevaluate your drivetrain parts and possibly downgrade to well-reviewed but cheaper parts. However, purchases online, buyer beware, there are increasingly fake parts being sold in fake packaging. Try to buy from reputable sellers.

I tried to replace my chain after only a few thousand miles, skipped, too late, needed to put the old chain back on and will replace the chain with the cassette. Since doing more standing climbing, higher chain tension, the chain wears faster than spinning up hills in lowest gear, and that changed the tooth spacing on the cogs enough, that a new chain didn't match.

On my old road bike, I was young and clueless about checking chain length/stretch, but was religious about monthly chain (hot-melt) waxing. And I rode a LOT. 16 years later and I notice the rear cogs are a bit cupped with burrs on the thrust side of the teeth. It's only been 75,000 miles. Original chain, cogs, and chainrings. But they were perfectly "worn-in" together. (And most of the mileage was on the flats, and later on mild hills, I always spun up back then.) Upon inspection, chain rollers are mirror smooth but dished like the side of a nuclear cooling tower. Replace chain. Replace crank and rings (2X to 3X conversion for hills). Deburr cogs and simply reverse them, you could do that pre-Hyperglide! Rode great. That bike is in storage but replacing that cassette will be difficult as they no longer make UniGlide cogs (no Hyperglide shift ramps), nor threaded high cog (no cassette lockring), and the 126mm spacing will not fit a Hyperglide freehub body and cassette. I think Sheldon Brown website has some hacks if I ever need.

I have the same drivetrain you have. All SRAM eagle chains will work. XX1 is the top of the line and should give you no problem.

Was the old chain an SX or GX, like I said before, those wear out extremely fast and can ruin your cassette. It's a well known problem.

The quick link to attach the regular is a flat top. Did you connect it facing the right way? Flats to the outside of your circle.

All Eagle derailleurs look bent inward. It's how they are. If your hanger is straight, don't worry about what the derailleur looks like.

The upper pulley on an eagle derailleur is narrow wide and well as the chainring. Due to its size, it's actually difficult to get it wrong. Double check though.

Make sure you've threaded the chain correctly into the der. This is more of a road bike problem but check anyway.

I'm going with my original post on this, that the older worse quality chain wore out your drivetrain. Those chains really should come with a black box warning.

Thank you.

That is very useful info, especially about the derailleurs being slightly bent inwards, intentionally.

Unfortunately I don't have a record of which chain the old one was. I posted a pic above; it just says EAGLE 352. I used to buy from a shop called Chain Reaction Bikes but they went bust.

At some point, when I start having trouble with my existing stuff, I will replace the whole lot. BTW, this is the cassette I ordered
https://www.amazon.co.uk/dp/B071G9Y74V
and hopefully at that price it is not counterfeit

FYI, most narrow-wide chainrings need replacement if you have changed a cassette due to wear. If you get two cassettes out of one chainring, that is considered reasonably good.

Their cumulative wear usually results in noisy operation, or in more extreme cases, a chain not releasing smoothly...same as it ever was. Component wear manifests as skipping on the rear, and typically as failure-to-release on the front, unless it's simply good and roached.

Hmm... thinking why... narrow-wides have every other teeth wider, so more resistant to wear, the narrow teeth are standard width. So my guess is this results in different speed of wear between narrow and wide, resulting in length mismatch at a very local level. Maybe not, as the wide teeth will still be pushing on the same width roller (and not the inner sideplates, hopefully), so perhaps the roller contact point is the exact same width as the narrow teeth. Curiouser and curiouser.

A properly maintained drivetrain will last a really long time. A cassette will last many years of heavy mountain biking if you don't run a worn out chain.

To say that a chainring will only last through 2.cassettes is like saying that your chainring will last 5-6 years with proper maintenance. I can live with that.