I think some people are underestimating that hypothetical team's ability and also Ainge's ability to surround a contending core with quality role players. Three of our best assets are Brown, the '17 Nets pick and the '18 Nets pick. If we can retain two out of those three assets, plus a core of IT, Hayward, George and Horford, we've still got a ton of talent, two young potential stars getting better and better every day coming off of the bench, ring-chasing role players and plenty of trade assets for the right opportunity at the right time.
In the OP's scenario, I don't think anyone in the East has a team that good both in the present and the future, including the Cavs. Can that team beat the Warriors? Probably not in 2018, but it's still a contending team with a ton of upside as two of Brown/Fultz/Nets '18 pick ideally develop into star players.