So one very good argument for going to 10 or 11 speed, rather than 9 speed, is the increased flexibility / choice / range of sprocket sizes available, off the shelf. The same argument can be made for size / range / flexibility / choice of the chainrings.
RR you keep changing the goalposts in an attempt to bolster your flawed proposition above. Bonus question: Can you elaborate on the "same argument" for getting 10sp chainrings (as opposed to 9sp)?
Starting position:
it's rare to find an 'off the shelf' 9 speed cassette with a big sprocket with more than 27 teeth
Then, when people point out there are plenty of 9sp cassettes with more than 27t, you insert 'road' before 'cassette'.
you won't get the latest greatest lightweight high end cassettes (Dura Ace for example) with 9 speed spacing, and a wide range, in an emergency, from a high street retailer, off the shelf.
Then you introduce the idea of 'Dura Ace' "high end" cassettes. The OP's bike is a Oz300$ bike with Sora triple on (I'm guessing). Why would he/she want a DA level cassette? And of course the reason you can't get those is because DA moved to 10sp ?13 odd years? ago. But
suitable 9sp cassettes are readily available, even from
HF.
If someone wanted the latest greatest Dura Ace lightweight gear, with a wide range, and 9 speed spacing, I wouldn't fancy their chances, of being able to source it easily.
Then you introduce 'wide-range' (not something the OP mentioned and by implication has no need of (looking to increase the chainring from 30t to 34t). So the points I made above remain valid. Of course you won't find DA around because that level is 11sp this decade.
a road cassette, on spec ( walking in off of the street, and picking it off if the shelf there and then ) is less at a typical high street type retailer)
Then you suggest that cyclists rush off the street and "on spec" buy a cassette. Really?? And why focus on the 'high street retailer'. I'd hope that, if not online, your LBS was the best place to sell you a cassette, and for those not able, to fit it.
A range of 9sp cassettes are available, readily available, to fit a road bike - so the suggestion that they are not makes (your words) "very good argument for going to 10 or 11 speed, rather than 9 speed" flawed. NB: see bonus question in Line 1!