The feature team has decided to decline this request after careful consideration. Thank you for the suggestion and all the feedback.
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.
SuphatraJohn Kerns, Jr. commented
I agree with the last few comments on this thread and hope someone revisits this item from the MS Teams team.
As was mentioned, if someone has signed into MS Teams at any point but no longer use it regularly, you are no longer able to send a chat to their SFB account. I agree with Jake that it would be beneficial if there was a way to search for a user and then select their SFB account instead of their Teams account so that you were sure that they would see the chat message in SFB.