This skips all the content before the previous expansion to give players the most modern experience from a quest design POV (and to make sure the players are connected with the current storylines). It also ensures new players have a shorter leveling ramp—Blizzard says it will be as much as 70% shorter than it is currently to get from level one to the cap.
Blizzard hasn't talked about what it plans to do after Shadowlands. I speculate that the longterm solution would be to do this every expansion—so in the next expansion, Blizzard might move Battle for Azeroth into the multitude of choices for new character-leveling experiences and push level-cap characters back down to 50 to start whatever expansion comes after Shadowlands—an odd hybrid of traditional MMO and a seasonal model common in plenty of modern multiplayer games.
A new army of the dead rises: to combat Sylvanas' assault on Icecrown, Bolvar Fordragon— the revered former paladin who took on the mantle of Lich King to keep the Scourge at bay—has raised heroes from among all the peoples of Azeroth to bolster the Death Knights of Acherus. With Shadowlands, pandaren and all allied races will now be able to become death knights.