That's a good question. Everyone has different obsessions and specialties so I don't think there will be any truly forgotten vacuums as long as there are serious collectors and enthusiasts. Sometimes we get questions about 100+ year old machines and there's usually someone on here who at least has a clue.
But I agree, I think the cordless machines will be the first to fade from memory. The batteries just do not last long. The Sharks in particular are especially filthy. I actually think of that Oreck/Hoover regularly lol, I just don't have enough room for another vac rn.
We'll have to see what the next generation thinks! 10 years ago late-model Hoover Elites belonged in the trash, now they sell for over $100 on eBay. My prediction is the Hoover Caddy Vac will eventually become a collectible oddity, and has already been moving into the forgotten category.