It might be easy enough to add a checkbox to the composer UI but space is a valuable commodity already so I don’t know how it could best be squeezed in.
IIRC, right or wrong, the Poll feature is considered to be a “power user” thing. i.e. not for the newbie, but discoverable to those that have learned a bit about how things work.
IMHO a Poll and discussion are not mutually exclusive. Have a Poll makes it easy to tally results without needing to count per post.
I agree with the restriction to alter options 100+% Changing the options after votes have been cast invalidates the poll making it useless.