By your definition, OS9 multi-tasked. Suspending apps in the background is cooperative multi-tasking. It's what OS 9 and earlier used, it's what Windows 3.1 used.
Do I need to go on ?
Please don't! I can see a lot of apps on OS X still being fully multitasking, but realistically if you haven't looked at an app for a while, why can't it suspend it to disk and make the RAM available AUTOMATICALLY to a program that needs it?
"Automatically as needed" is the key. Most users have no idea what will and won't need memory, so let the OS decide, after all it is in the best place to make such a decision. Even geeks don't have enough info at hand at any given moment to make that kind of decision, regardless of what they might think.
Yes, it is. Sometimes when the GF is yelling asking me to come downstairs to help her with something, if the page I'm reading has 5% left, I'm going to finish. If it has 50% left and I know the first 50% were long, I'm going to go help her.
Quick. Visual. Cue.
Scrolling ever such a small amount, to satisfy this almost irrelevantly proportioned use-case (versus day-to-day use) is hardly a big deal. And if the 50% was that long for it really to matter, you'd have noticed that fact the most recent time you scrolled, wouldn't you?
Realistically the scenario laid out by you will happen less than 0.5% of the time, it isn't really worth wasting any time giving thought to. Remember the 80-20 rule.