Yes, and you can play those exact games on the original Bnet servers just like they are.
If you want to play D2R with completely re-made graphics, cinematics, engine, features, then you will have to buy it separately.
Someday it will decrease in price or there will be a sale. After all D2 (2000) was around $60 new and LoD was $30 or $40 new. As you noted, they are now $10 each.
I am not sure why you would expect owning the old version to give you a discount on a new game. Consumers don’t get discounts on re-releases of music or movies for owning the old ones.