I wonder how much the compatibility between sets (being able to snap blocks made in the 80's to ones from a set today) affects that, along with them having more of a "repeat use" than something like a baseball card or comic.

I think that's the distinction that matters. You can collect it, and it stops being sold, now somebody somewhere wants your set in brand new condition so they can play with it, cause well they love Lego's and you've got some rare set they wish they had.

The lower bound for the price of a collectible Lego set is what you could expect when sold solely as a cheaper, slightly outmoded alternative to a new Lego set. That's better than baseball cards and the like, but still easily offset by the cost of storage and handling.

I have Lego bricks that are from the 80's and they snap together with brand new ones just fine even after being used by my parents through their childhoods and through my own

The Lego Group has some pretty tight tolerances for what's acceptable in a brick. For being a toy, Lego bricks are created to incredibly high standards.

Yes, but this time it's different.

That's what everybody inside the bubble thinks. That's what they must think. It's what makes the bubble work. The bubble must always contain some forgotten ingredient that makes it special. Something that people do not remember about the past bubbles. Then nobody inside the bubble can prove that it will burst and if it's not provable maybe it won't burst.

I wonder what difference intended collectability has on the value if any.

All three tried to exploit those to inflate the bubble. Given the strategy seems more toys first than limited platinum collector's edition - which doesn't really have much demand over the baseline. I would expect an overleveraged by speculators market to collapse eventually but far slower - akin to the decline in antiques as many new buyers shun them as white elephants.

The thing with collectible markets is that usually price collapse trails far behind transaction volume collapse, so that the few examples of complete price crashes are just the tip of the iceberg. Apparently even the most professionalized traders would rather hold on to imaginary riches than liquidate their stocks for pennies.

Also BTC.