There is a lot of "moral reasoning". The fact is that if I buy a new OS version, and it _intentionally_ breaks my software without any good reason, then I would be annoyed enough to annoy them back. Note that in a UK small claims court things are stacked heavily in my favor. Apple would have to make an appearance in court, which in itself is more costly than what the matter is worth to them. The other problem that Apple would have is that they would have to come up with some very expensive experts if they want to contradict anything I can come up with in a court. Then there would be the small matter of front page news (can you imagine what TheInquirer would make of that)?
So where is this moral reasoning? All you've said is you'll be annoyed if you buy something that you knew wouldn't support some existing software, and it unsuprisingly doesn't support some existing software. You're also saying you'll win not because you're right, but because Apple won't be able to air a defence (which is actually morally bankrupt on your part, to be honest).
Under what section is Apple required to continue supporting existing software in a new product?
How much do you expect to get in compensation? The cost of the OS I presume, in which case why don't you get a refund?
The answer to that of course is because the OS wouldn't be faulty providing Apple doesn't claim it can do something it can't (which they won't). This means the onus would be on the buyer. You'd be at fault for buying something you knew wouldn't do what you wanted it to.
The fact that you wouldn't even try to get a refund would also mean Apple wouldn't owe you anything because they won't have refused the refund. Think about it for a minute, Small Claims Courts are there to resolve existing disputes - whilst you're trying to start a dispute in the Small Claims Court.
How are UK Small Claims Courts "stacked" in your favour? You're required to know what you're talking about, the legal basis for your claim (which you still haven't provided) and you may actually be required to provide an expert's opinion. That's right, yours may be worth **** all.
Lastly, if you truly are prepared to go down the route of litigation, would you mind providing some details of when this will be - so this thread can be dragged up.