I don't quite see it this way. The airlines are extremely lucky to get away with it.
Let's say I purchase a seat on an A380 to go SYD-SIN. If that is downgraded to a 737 then I am no longer receiving the service I paid to receive.
And I don't quite understand how airlines continue to get away with their meaningless fine print that somehow allows them to screw you when they feel like it.
I do see your point John, I guess it comes down to one's definition of "service" that one has purchased - from the bare bones "transportation from A to B" through to F&B offerings, specific seat type - eg: flat bed or perhaps IFE screen in Y (thinking of the 332 domestic situation). I can see arguments either way.
Interestingly along these lines, QF's emails to transcon customers (detailed in another thread here) suggesting an old config sub with lack of entertainment options and an apology etc seems to suggest that at least one part of QF at least appreciates you expect a certain standard which they're not meeting, yet as you say they do fly 737's on transcons (ugh!) yet the bean counters would say "so? it's a transcon seat" - they're not sitting in it!
The reality can be, specially on domestic flights, that for any number of reasons from aircraft subs, mechanicals or simply low bookings that aircraft subs and changes do happen. I don't know if it's quite "when they feel like it" - but I guess you could say a sub or cancel due to low loads could come under that general definition.
I would think the end result would be that is one feels they have not received what they expected (say a 330 sub for a 737 on a long flight) they could ask for some sort of compensation from the airline - which they may or may not get. I'd submit the longer the flight, the better the chances.
The same applies really if you're booked on a flight and the aircraft has a mechanical or other issue and is subbed late.
My point was that nothing is reall guaranteed in the airline world, most specially on domestic routes where things can often change. Of course with QF's domestic fleet being basically 330's and 73H's now there's not going to be much variation, excepting the 330 Business Suites and perhaps the 73H's with the ISV and sky interiors that are out there.
I' say international can be more consistent, but we've even seen recent changes on LAX flight with 380's subbed for 747's - with most affected pax being those in (paid) F, then upgrades and those in J who maybe get an older config. I would say though, putting the F pax aside, if you were on a 380 in J and you're subbed to a refurb 747, also in J, is the difference THAT much to warrant a complaint? I guess that's up to individuals to decide.
mostly rhetorical questions here