I'm not a Smart hater, but he's not the third best player on the team. His intangibles are very valuable but so is efficient scoring at decent volume, and this year, Smart vs Tatum:
Tatum: 17.6 PER, 4.8 WS, 2.8 OWS
Smart: 9.6 PER, .9 WS, -.8 OWS
Now, clearly the advanced metrics are missing something quantifying Smart, however, there is obviously an argument to be made that Tatum is already better. In a league where the Warriors win titles by outscoring anyone and the Cavs have a poor defense but are still a clear top 3 team, Tatum is extremely valuable to keep pace, just as Smart is to slow them down.
Even at just 19, Tatum is playing like Otto Porter this season, and Porter just got a max deal. Smart, based on his numbers this year, will be lucky to get half a max deal this summer.
In fact, though this isn't the argument being discussed, when Tatum comes off his rookie deal, he'll likely get more $$ a year than Smart ever will.