Hate to be the bearer of bad news. This item has been declined. There will be no work to hide or remove the reply button.
As many people have commented, a big ask is to help improve how people distinguish between giving a reply and starting a new message.
For that the focus will be on this item. “Better visual delineation between `Reply` and `Start a new conversation`”
http://microsoftteams.uservoice.com/forums/555103/suggestions/19224826Adam Chopko commented
Threaded conversations are great, but agree to change the UX so "Reply" doesn't appear under every single response.
It looks like some people in the thread below have figured it out — we actually have this. You can start a chat both ways; the system will automatically recognize if this person is on Skype for Business or on Teams.
Thanks and let me know if you have any questions.
This feature item is still in the works. We will keep you posted as we have new info from the feature team.