I hear what you're saying. But offense only bigs still have value when they are relatively young. Just look at Kevin Love, he was a key piece in the Cavaliers Championship team. And defensively you could argue that Love was a liability with his lumbering feet. Of course the Cavs had Lebron who was the main piece but their big three was Lebron, Love, and Irving.
Can the Celtics win with a big 3 of Tatum, Brown, and Toppin? Don't forget we also have Kemba Walker so we kind of have a big 4. You can hide defensive weaknesses, but if you don't have the scoring down the stretch you can't win games. And I feel like the Celtics lack any kind of big man scoring quality and Toppin would provide that. There were stretches against the Heat when the Celtics went ice cold from the perimeter and they had no big guy they could just give it to score some easy buckets inside. Meanwhile Bam Adebayo got a ton of easy scores inside against us based on size alone.