My question, quite simple really - is changing just a chain the better option than waiting for everything to wear down until non-rideable?
Actually I'd say it depends on the rest of your drivetrain.
I used to use XTR cassettes (c£130) and KMC X10 SL chains (c£40). If I changed chains when they reached 0.75 to 'save' the cassette, I'd need a new chain every 3 months or so. So I'd spend £160 a year on chains, and nothing on cassettes. As it was, I changed a chain a little too late, and it slipped, put the old one back on and it lasted 2 more years. So I had a bill for £170 after 2 years, whilst I'd have used £320 of chains in that time, and probably still needed a cassette eventually anyway.
XX1 cassettes shift the balance the other way - chains last longer IME, they're only £30, and the cassette is >£200, so changing chains makes more sense.
If you run cheap cassettes (XT or less) personally I'd run the whole lot into the ground and change it every year. Otherwise you spend (say) £80 on chains, to preserve a £40 cassette.
The only slightly caveat is that a 'new' drivetrain does sound and work slightly better, if you run everything into the ground it can be a little ropey toward the end of its life.