Why did you copy and paste a segment of a post I wrote?
anyway it is true, they will.
Because the 1.17 update based on the information we have been given so far and what ibxtoycat has shown,
we will not have deeper terrain and all their structures on existing chunks from 1.16 or earlier.
The deal breaker is you'll either be going out further to generate those newer chunks,
or start a world from scratch.