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The external dirt isn't the stuff that's grinding the chain away though. If you never flush out the wax from the the rollers and pins then you're leaving all the tiny metal particles in there that are the cause of the chain wear.
I've never tried hot water cleans on my chains though so don't really know how effective/ineffective it is. £15 crockpot though means I'll likely never bother to find out.
The idea is the hot water will wash off the dirty wax on the outside of the chain while leaving the wax that's still clean inside of the rollers etc etc where it is.
Then you reapply more clean wax on top of that once it's cooled a bit.