How does the new Backup and Sync treat existing Google Drive users?
Well, looks like unlike what all news site claimed, it does not change the old behavior, but adds something else.
The new feature is a semi sync as the new multiple folders do not use My Drive:
This means they won't sync to your other computers, only to Google Drive's webpage.
In other words, if you have multiple computers, then the only folder to sync between all of them is the good old Google Drive folder.
The only exceptions are files and folders on which you right clicked and chose to add to My Drive. Yes, just like files and folders that other users shared with you.