That doesn't sound like a reason for it to be hard - just provide a way to mark tabs as "don't suspend". I have only a couple I would put in that category, and I really wish Chrome would stop burning up all my resources and battery on the rest (often it's not a specific tab, just the "Browser" process, so I can't do it manually).
I'm pretty sure the amount of users that actually like this opt-out behaviour (everything gets suspended by default) is a minority.
Instead what they choose to do is not worry about memory until it you actually run out of it. I don't mind my applications taking up 90% of my memory if that keeps everything fast and I don't have to worry about keeping track and thinking about what I actually want to suspend and what I dont't.
If you really do mind, just run any of the plugins that implements your exact required behaviour (like the great suspender).
