Dynamically update contacts in send list for broadcast emails

If on Monday I schedule a broadcast for Friday to go to all contacts with a specific tag, that list of recipients is locked. If one of the contacts is removed from that tag on Wednesday (perhaps because they clicked on an opt-out link), they should also be removed from the Friday email.

I understand that this can be done in an automation but it is unnecessarily complex to build a whole automation just to pre-schedule an email! 

You don't have to change the broadcast functionality entirely. Just add the option to choose if you want your receipent list to update dynamically or stay static (the current default).