Those are some really good points. Now that you mention it, I have that feeling of going in and pointing out the flaws of their logic. But I really should just leave them in ignorance,they'll have more fun that way. I guess I also feel like trying to prevent them from thinking that I'm leaving because I can't power game in 5e (as if), even though I wasn't power gaming to begin with (at least not in that group).
Only two of them (out of 6) I'm friends with outside of DnD. So maybe a message is the best way to go.
I would send a polite thank you, and move on.
Doing the above will convince them that you're the antisocial powergamer they seem to think you are. It's a bad move. Also, it's going to come across as rude and condescending.
This is true even if you do not mean to be. I'm not pointing this out to be mean at all, by the way.
A polite note with a thank-you attached is the way to go.