Yep, and the thing is you will be able to obtain artifact appearances bar 2 post Legion, so they will probably "stay in your bags" just lose the powers. Probably on pre patch as a part of class preparation for next expansion.
Legendaries might lose power as well, first as a part of levelling up (similar to WOD ring / MOP cape), and some time later they could completely deactivate these even inside level scaled content, as they did with pre-WOD tier sets since they didn't want to go rebalance them for twinking (and some of these referred to spells no longer existing or drastically changed).
But I don't think you'd lose them as a "transmog item".
7.0 made all classes pretty dull as it turned out we would be balanced to have interesting gameplay once we were with our artifacts and legendaries. Come 8.0 I think they'll have to add some salt and pepper into the baseline design of the classes instead since artifacts won't be staying. Mind you the legendary idea will be carried over in another form (that necklace and so on) so I'd expect our characters to still feel a bit stale during leveling.
Most of the important artifact and legendaries passives will become baseline.
I don't really like the fact that each class will keep the same gameplay from 8.0 to 9.0 though (no sets, no major talent tree evolution, no legendaries, no bonus anywhere).
I still use old leggies cloak ring in time worp dungeon so it will be still useful
basically the specs that are completely dependent on certain legendaries to be viable will be broken until at least 8.2, probably 8.2.5, then they will fix them and people can play the class they want again