As someone mostly disinterested... I don't sell supplies, but I've used Mylar, as well as sleeves and toploaders and... Pretty much most hobby supplies that have become available since 1974/5
There are a few benefits beyond how long they last and how they don't interact with what's inside them.
They are a lot clearer than even the new penny sleeves, which are much better than some of the older ones. If there's something that has better clarity than Mylar I haven't seen it. (The first were polyethylene the newer ones are polypropylene. Mylar is polyester. All three are ok archivally, but Mylar is considered to be the best. )
It has a lot more natural stiffness than the penny sleeves. (At least the ones I used had a lot more. ) Think more like cardsaver stiffness. So if you aren't constantly handline the cards, you may not need toploaders.
In a "worst case" scenario. Mylar sleeve with enough room- like a T206 in a sleeve sized for 57-? Topps can self seal to some extent. Which could keep them from water damage. Mylar melts at almost 500F polypropylene at 320F, polyethylene at somewhere between 248-356 F depending on the type. The sleeves were almost certainly the lower end of that.
So A Mylar sleeve near a fire so the sleeve/card gets hot but not hot enough to burn, will probably be ok. The others may melt and laminate to the card.
Even if you're inclined to replace the holders every few years. I'm not, and with a bigger collection, just don't have the time or money.
With Mylar, replacing the sleeves isn't a "must do" sort of thing.
All that being said, I am the sort of person inclined to leave a card in a holder for a very long time. I think I stopped using the pages from 1974 sometime in the early 90's. They were 9 pocket side load, with no extra space so the two rows where the opening faced each other got worn on the edges. I do still have a handful of cards in PVC pages, and more in the first generation of non-PVC pages and some holders from maybe 1980? 81?
None have suffered any damage, aside from a couple I may have creased slightly removing them from the pages.
I've had both penny sleeves and Mylar in a situation that was pretty abusive. I forgot a couple cards on a windowsill...a sunny windowsill... For a couple years
The penny sleeves were too brittle to handle, the Mylar was still perfect.
Archivally, changing out the holders periodically is probably fine, but there's always the chance of damaging the item while handling it.