Oh, and...
...there are *no reasons whatsoever* to why you shouldn't be able to upgrade from an ISO. Every single one of the public MorphOS releases (v1.0+) came packaged as an ISO, and it was only on one or two occasions (1.4->2.0?) that you were recommended to do a clean install instead of upgrading. The only downside (and it's really a minor one with today's Internet bandwidth standards) is that file size may be bigger with an ISO, but the upsides are so much bigger. Clean install or upgrade - it's a matter of how the installer is set up (and then how the user decides), *not* if it's an ISO or not. With a properly put together ISO you will have a choice, without it you never will.

Absolutely, that should go without saying, but wasn't my point. Maybe you were so eager to disagree with me (I am an OS4 user after all) that you failed to understand what I was actually saying.

I'd like to be able to download an upgrade package, double-click it, and it goes ahead and upgrades my installation. Not to have to mess around burning CDs or mounting virtual CD devices. Update 3 worked like this for me, and I would much prefer future upgrades to work that way too rather than being ISO-only.
And, as for your previous point, yes, it's a shame the issues with the update were handled the way they were; I'm not denying that. As you pointed out, other systems have issues with updates too, and it's not unique to the Amigoid world either. The company I work for has several parallels with the current Amiga world: custom hardware, userbase measured in the hundreds or maybe low thousands, software updates maybe once or twice a year. There should only be 2 different configurations of our machines in the field, and we thoroughly test each software release on both, yet issues still crop up because: a) we have no control over what the user changes on the machines, and b) sometimes the updates show up obscure firmware or hardware bugs on specific setup and usage combinations. These things are just expected, and happen no matter how much validation we do on our systems.
What might even scare you is that these machines are used in hospital labs for testing patient blood samples prior to and during surgery! That's a far more regulated and tested product than some unheard of hobbyist OS, but still suffers from the same issues.