They already were. Despite the fact that they were once the pinnacle of magical ability of Azeroth, they have gone by the wayside since they've been in hiding. The Moon Guard was easily beaten by the Nightborne in Suramar, and they didn't really add anything to the campaign later on. The Highborne of Eldre'thalas have stagnated themselves for 10,000 years, which has made them look outdated in the eyes of other elves. In Azshara during a Horde questline to stop the Alliance's effort there,
Andorel Sunsworn mocks the highborne, saying that they have wasted potential and outdated magic - he uses some of their outdated spells against them in the form of "hacking" their arcane familiars, which he points out were outlawed in Quel'thalas 1500 years prior for that very reason - though obviously the Kirin Tor, ogres, and even Nightborne heavily use arcane familiar to this day. Even the other night elves distrust or dislike the Highborne, not really allowing them to practice as much magic as they would like (at least as of Wolfheart). even in that book, they were afraid of stagnating magically if they didn't practice magic in the woods (which Malfurion had forbidden).
We have seen a few Highborne, of course, since Cataclysm, even a few in Legion itself. However, very few are shown to be important. Two of them are simply looking for the ghosts of their parents at Nar'thalas, and one stands in the entryway to the mage order hall - though he has no lines or services. We certainly see a lot of ghostly Highborne around Azsuna - Farondis most of all. We do go inside the academy and learn a few things about wands and spells, which Khadgar has us repeat in world quests because they will "help us against the Legion" due to their renown.
However, the most advanced and powerful versions of Highborne are now Blood Elves and Nightborne. High Elves are few in number, and half of them seem to prefer being rangers. Highborne are around, but lore-wise they aren't even allowed to practice magic in the city they live in.