I use scheduled messages pretty frequently (both in Textra and Boomerang). Boomerang has an option to send the scheduled message if there's no reply. I'd love the same in Textra. I imagine it working like this:
1) I schedule the message and check a box "Don't send if there's a reply"
2) When it's time to send, if there's a newer message from the contact, don't send the message (marking it as abandoned) otherwise, send the message.