I think that all your objections boil down to "instant gratification does not harness the brain's reward system as a motivation to achieve difficult things and

I think that all your objections boil down to "instant gratification does not harness the brain's reward system as a motivation to achieve difficult things and improve general intellectual/physical/social abilities". It is true, but somewhat orthogonal to other problems such as how to reach happiness. As an intellectual leaning person, I tend (like you) to link my own happiness to greater ability of my mind, but that is absolutely not generalizable. For some other people, the notion is true but only regarding "to improve physical abilities", and they don't care about the other parameters. etc. Moreover, the highlighted problem of instant gratification thus applies not only to sexual needs, but also to feeding needs, for example. It would be interesting to compare foraging/hunting societies with ours in this regard.