We're also hitting a time when 90s kids are reaching a place in their life where they have reached a point in their lives where nostalgia and disposable income meet, and so many are going back and getting the stuff from their childhood, including comics, increasing demand and hence prices. When I started collecting comics seriously in the mid 80s, 70s Bronze Age books were dollar bin fare. I got the entire Fourth World run by Kirby for 50 cents to a dollar each, including the first issues and first Darkseid around the time Legends came out as that was my first real introduction to those concepts (outside of a stray Secret Society of Super Villains issue) because I wasn't much of a DC kid. Nova, Ms. Marvel, Spider-Woman, all 1st issues you could get for a buck or less. They had zero value in the eyes of collectors, until 70s kids like me reached the point in their lives where nostalgia and disposable income met to create more demand for those books and give them sustainable value. And dealers and collectors used to scoff at those Bronze books books ever having, letting alone sustaining value. But it happened.
Back then early Barks Ducks commanded prices on par with or greater than Silver Age Marvel keys, and outpaced most of the Marvel keys in the Overstreet lists of most valuable books in the annual guides. However, demand for the Marvel books kept rising and demand for the Barks stuff plateaued and eventually dropped as the folks who had nostalgia for those and disposable income aged out so the demand wasn't there to sustain that level of pricing and collectability for those books. All comics, no matter what the age, content, or rarity, are only worth what people are willing to pay for them and only have value if there is demand for them. If demand drops, book values will drop. Since super-hero comics have come to dominate collecting, the demand for those is higher and the prices seem sustainable, but if that demand goes away, so will their value. A few have achieved pop culture icon status, so will be more resistant to devaluation (your Action #1, 'Tec #27, Amazing Fantasy #15 etc.). Resistant but not invulnerable to it.

And 90s books are no different. Demand is rising. Prices on some will rise concurrently and will sustain as long as demand does. That's the cycle of comic collecting. Books that were giveaways or could be had cheap, shoot up when demand (real or perceived) goes up. Books that commanded big prices drop when demand goes down. Some older books (Golden Age, not Silver) are legitimately in shorter supply, so need less demand to sustain value. This is true of some genre books outside super-heroes as well (romance and pre-code horror in particular) where there is less demand for the books than super-heroes but because supply of them, particularly in higher grades, is also smaller so it takes less demand to sustain market prices they are achieving (and that demand is grown as some collectors turn away from super-hero collecting for many reasons including being priced out of keys currently).

But the bottom line is there is no inherent value that is sustainable in any comics. Those prices will rise and fall for most books is direct response to demand levels. The demand can be real or perceived (if someone perceives a book being in demand or anticipated demand will rise for whatever reason-1at app of character in other media is what drives a lot now-then prices will start to rise as people try to pick up copies for themselves or to resell), but it is that demand that will fuel the rise or fall of prices, with the rare exceptions of those handful of books that move into iconic status and are seen by even folks outside the hobby as sustainable commodities to be trades on (again your Action #1/Tec 27/AF 15 type books) but even those are not invulnerable to changes in demand on their pricing, there's just much less chance the demand for those goes away for various reasons.

So no book has an inherent sustainable value, and books once perceived as worthless can, and do on a regular basis, gain value and some sustain it long term. As we get older, more books we have long assumed have no value will be among the risers regardless of supply levels because generational demand changes will be what is fueling the demand for those books. And things we long presumed had value will drop, because generational interest in those books will dwindle as those interested in them age out and die off without the interest being passed on to younger generations (even if those of us in that older generation don't want to admit to this or deny it because it plays into fears of mortality or even worse of becoming obsolete).

This is the way.

-M