Hi, and welcome to Azeroth!
Ayup, it is definitely the intention that you move on to Shadowlands with only a little bit done in BfA, and long, long before finishing the story.
Remember, each expansion takes two years. While you can certainly play through one is much less than that, Blizzard consider it a bad idea to have new players spend a year or two properly playing through all the past expansions before catching up with the game.
Remember also, at least you are seeing some of BfA. You are not seeing any of the other expansions at all.
A new expansion is not a new game, but they structure it so that you don’r need to play any of the other expansions first, and can start straight into the new expansion.
So you will be level 50 and heading to Shadowlands way before finishing BfA, and that is normal and expected.
Will it be worth returning? It certainly won’t be required, but there are several reasons people do come back to older expansions - for the zones, for the story, for transmog, mounts, pets, achievements.
If you stay with the game, you will probably start creating “alts”, other characters of different classes and races that you play. You will probably want to send each alt through a different expansion, to get a fresh experience.
In the case of BfA specifically, you might want to spend some time there after hitting 60 to earn the Flight Master’s Whistle and, later, Flying itself in Kul Tiras. It’s nice to have.
!