Slips will, in all likelihood, soldier on until the skirt is entirely obsolete as a style, and in the meantime will dumb down in style the way that everything else has been being dumbed down. Thus, expect fancy flourishes like lace and mesh to go the way of the dodo.
I don't think they'll ever disappear entirely as would likely happen if the skirt as a concept (and especially the dress as a concept) wink out of existence. There will always be the ones who refuse to capitulate and join the denim wasteland. But -- expect slips and skirts to become "boutique items" in the next few years.
I own several, in various weights, for different times of the year, and it's only the older ones that have flashy embellishes like lace and mesh. Unfortunately, because they're older also means the elastics are beginning to fail and -- as you've noticed -- new stock is no longer available because nobody cares any more. A look at assorted catalogues will back that assertion up. Finding anything "nice" is getting to be darn difficult, and I find myself "online" shopping in what amount to speciality boutiques that deal in historical garments (which is fine as I gravitate to Victorian/Edwardian styles -- but causes problems because there's nowhere to wear such rigs without attracting "undue attention" and, recently, scorn).