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Changed my chain ring, chain and cassette 18 months or so ago
Sensibly, I bought 2 chains to rotate them and maximise life of cassette
So of course new chain has sat in garage ever since
Measured original one - it’s over the .075 (or is it .0075?) wear limit. Stuck new chain on anyway and it runs and shifts fine (no chain suck or anything) but is rumbly in lower gears (not surprisingly)
So -
Stick with new chain, i suspect it will get quieter with a few rides ; or shove knackered one back on and ride chain cassette and ring into the ground?
I'd say you've waited too long. When the new chain goes quiet, you'll know you've accelerated wear on 0.75%.
If it were me, I'd put the old chain back on and run it all to destruction, then keep the new chain for the new cassette and chainring.
I'm assuming the cassette is the most expensive part of all that and what you're looking to protect but it sounds like it's already worn.
If it's still close to the .75 mark then I'd run the new chain. When I was doing the hire fleet that's what we'd do and that was based on a few years/multiple bikes "testing" sample.
2 chains to a cassette.
2 cassettes to a set of chainrings.
We didn't "rotate" in the way that your post suggests. Keeping track of over 100 chains for more than 50 bikes would have been a step too far 😂
If it's shifting fine then stick with the new chain.
If the new one works, stick with it.
When it gets to .75%, try another one. You probably still won't need to replace the chainring, and you might get another chain out of the cassette if you're lucky.
I've never been a fan of the "run it into the ground" approach, because I find that once they get much past 1% the drivetrain noise is unbearable. Chainrings are expensive, and should see out plenty of chains.
I also don't both with "rotation": just fit a new chain before .75% and bin the old one.