It's certainly possible to pay for such a thing today; many of our friends in Fortune 50+ companies have access to such things. You are right that we mere mortals with dreams of a tiny coffeepot IoT empire over HTTPS must hope for the post-Let's Encrypt era that the cost of such delegating certificate authority certificates drops in commensurate to other certificate types.
Where can I get a certificate with the CA flag set for mydomain.com? I did not know this was an option for mere mortals.