`some-command | grep pattern` and `some-command | rg pattern` both work fine. You can chain `rg` commands in a shell pipeline just like you do `grep`.
What the GP is suggesting is that their most common use case for grep is recursive search. That's what ripgrep does by default. With `grep`, you need the non-POSIX `-r` flag.
The other bit that the GP didn't mention but is critical to ripgrep's default behavior is that ripgrep will ignore files by default. Specifically, it respects gitignore files, ignores hidden files and ignores binary files. IMO, this is what most people mean by "ripgrep does the right thing by default." Because ripgrep will ignore most of the stuff you probably don't care about by default. Of course, you can disable this filtering easily: `rg -uuu`. This is also why ripgrep has never been intended to be POSIX compatible, despite people whinging about "backwards compatibility." That's a goal they are ascribing to the project that I have never professed. Indeed, I've been clear since the beginning that if you want a POSIX compatible grep, then you should just use a POSIX compatible grep. The existence of ripgrep does not prevent that.
Indeed, before I wrote ripgrep, I had a bunch of shell scripts in my ~/bin that wrapped grep for various use cases. I had one shell script for Python projects. Another for Go projects. And so on. These wrappers specifically excluded certain directories, because otherwise `grep -r` would search them. For big git repositories, this would in particular cause it to waste not only a bunch of time searching `.git`, but it would also often return irrelevant results from inside that directory.
Once I wrote ripgrep (I had never been turned on to `ack` or `ag`), all of those shell scripts disappeared. I didn't need them any more.
My understanding is that many other users have this same experience. I personally found it very freeing to get rid of all my little shell wrappers and just use the same tool everywhere. (`git grep` doesn't work nearly as well outside of git repositories for example. And it has, last I checked, some very steep performance cliffs.)
Some users don't like the default filtering. Or it surprises them so much that they are horrified by it. They can use `rg -uuu` or use one of the many other POSIX greps out there.