Anybody claiming that movies peaked and have been on a decline since the 80s really needs to shake off the nostalgia. I mean, really. There are great movies every year, including successfully franchised films like your beloved Indys and BTTFs. Are you forgetting The Lord of the Rings and Nolan's Batman movies so quickly? Even Pirates of the Caribbean?
These movies are all in the same boat. The only difference is you're not 16 anymore.
I have to agree whole heartedly that all movie memorabilia comes with an expiration date. Unless the movie was a landmark of the genre or medium (Toy Story is all that comes to mind), its value will diminish rapidly with its popularity. No movie is popular forever, no matter how good it is.
Many were not making blanket statements about the films of the 80's ( or any decade) as each has had and will have some gleaming gems, in amongst the piles of coal. And because film is SO subjective to personal opinion, you will certainly find people out there that have found (or will find) some of your examples as mediocre. It just happens-- whether it be film or any other art form.
Few are the examples that will transcend time.. Those that do have become (or will become) the classics. But with so much dreck being quickly churned out today, the gems (sometimes smaller, indy films) can get lost in the quagmire.
And there is certainly no question that some of the worst films ever can be made to look exceedingly good, in well edited trailers.
And as far as no movie being popular forever (a rather broad term).. i disagree. One only need mention a film like
THE WIZARD OF OZ-- 70+ years old, still going strong and instantly recognized by toddlers today and loved by most who have experienced it, time after time, to show that some films will remain very much, the cream of the crop.