Nostaligia is often a huge part of what forms the perceived value on stuff like that - the happy meal toys that I know of which can still garner a pretty decent price are some of the old handheld electronic games, they weren't particularly good but were popular at the time because they already had nostalgic value when they were released (I got a couple, lol, cos I loved nicking my brother's Oil Panic when I was a kid 
), and so now there is more than one generation of people that have nostalgia for them (parents bought into it at the time based on their own nostalgia, and the kids that played with them also now have nostalgia for them). That sort of toy is now produced purely to tap into the nostalgic market these days, but with something like a Snoopy toy, I suspect the prevalence of such toys in general prevents a happy meal version from ever really climbing in nostalgic value.
If something is only ever produced to be held in a folder and looked at, or kept sealed in wrappers, or put on display etc, where will the nostalgia come from in 50 years time? No one is ever going to go "oh yeah, I loved looking at that thing when I was a kid, I wish I had some to look at now", JMHO. I can see some mild interest in "so that's what washing powder boxes looked like in the olden days", but if that has interest and value to someone, it's going to be a real packet over a plastic replica that would have any real value (again, JMHO - just like it's the old, original biscuit tins people like and value, not the new replicas). People tend to need to have memories tied to it that are greater than the momentary excitement of seeing what was inside a packet. The kinds of "don't touch" collectibles that genuinely maintain (or increase in) value have more than nostalgia attached to them - things like rarity and quality craftsmanship.
Stamps and cards are probably one of the few things that acheived value when you couldn't really do anything with them other than put in an album, but even stamp collecting has diminished greatly because postal services started producing things purely to collect, thus taking out a significant part of the hobby for many collectors - the thrill of the hunt. Plus there is / was other aspects to a hobby like that (eg getting stamps from all over the world can be fascinating).
I could be wrong, maybe I'm just not seeing the true value of these things, maybe one day I'll regret not hangng on to my Looney Toons tazos, or the full set of fluro Dragon Ball Z tazos I had, but I doubt it.
