From the vocalness of the self-selected group who post about T1 and hypothyroid so frequently, I might assume the percentage was a lot higher too.
I do not think those who post frequently to online message boards are typical of the larger community.
I write all the above, knowing that I am both T1 and hypothyroid. But if I didn't have those attributes I wouldn't be posting in this thread, so again, a lot of self-selection going on in at least my posting here!
If my experience makes you feel any better, I was diagnosed with T1 as a kid and hypothyroid in college just a few years later. And no, nothing "new and bad" has happened since.
There's nothing wrong with letting your mind sort of appreciate the implications of a constellation of autoimmune diseases all connected with each other, and to let this guide your thinking about them and their relation. But don't think that there's a clock ticking and a new one will come along "every X years".
In Art Spiegelman's "In the Shadow of No Towers" there's a common thread about "waiting for the other shoe to fall". Spiegelman does a lot of critical self-analysis and it helped me deal with many events in my life (even if maybe Spiegelman hasn't completely moved on yet himself!)
I was diagnosed with Type 1 first, then Hashimoto's about 2 years later. About 5 years ago I also was diagnosed with vitiligo. So I've got the trifecta going! My wife tells me if I develop another autoimmune condition she's gonna have to trade me in!