New cassette
 

How many miles is a road cassette (9 speed) generally good for?

It depends on how much stretch you ran the chain with, and on what the cassette cogs are made from, and your riding style and terrain. Assuming the chain was never stretched more than 1/16 inch/foot, and assuming the cogs are good steel. I would expect somewhere between 10,000 and 20,000 miles for the cassette when riding flat lands and rolling hills.

The above assumes you change your chain at the 1/16" per foot elongation mark each time one reaches it. So, to get that mileage from one cassette you will need a bunch of chains.

My 18 yr experience is that one cassette will last two chains. Your mileage will vary but the two chain rule works for me. My mileage is
in the 3-4k miles for chains and hence 7-9k miles for cassettes in 8 or 9 spd. Only have about 10 k miles with 10 spds on two bikes
but one cassette started skipping in the mid range at ~8500 miles and was replaced last week (on new 3d chain for about 1500 miles
without problem til past 2-3 weeks.)

Your post is full of great advice. Also I would add a clean drive chain last longer than a dirty one. Dirt, grime, and road debries are chain, and cassette killers. IMHO.