That 2 big lineup sucks and Smart has earned the start. Him beside Kemba so that Smart provides the playmaking that Kemba doesnt.
There is no statistical basis for that statement. In fact, the 5 man unit you suggest sucks, Thompson, Theis, Tatum, Brown, Walker have not been on the court together this season according to NBA.com stats. Your proposed unit has not either. Tatum and Walker overall have not played together this season from what I can tell.
The line up of Thompson, Theis, Brown, Smart, Walker (still a 2 big unit) has played 28 min and is +13. That does not suck and I don't think putting in Tatum for Smart will make it suck. The 3 man unit of Thompson, Theis, Walker has played a total of 30 min and is a +18.
I don't disagree that Smart has played at a starter level but he can help the team off the bench also, will play starter minutes, and will still see plenty of important minutes. We will see how they decide to play this. I am just offering my opinion with supporting statistics, where they exist. I am sure you will see some of your proposed small line up also, just not that much.