Porzingis is a better basketball player than Smart. Smart is a great defender, but he was less great last year, and as much as his IQ defensively is a positive, his decision making on offense still is not.
The bottom line is that we had three point guards on the roster. None of them was an impact playmaker on offense, while they can all shoot from 3, none was a catch and shoot sniper, and somebody had to go. They tried to make it Brogdon and couldn't. I'm perfectly happy keeping White over Smart. He was better last year.
Grant was a casualty of the new CBA. Jaylen, Tatum, Porzingis are 100M, and White, Rob, Brogdon are another 50. You can't lock yourself into 13 more for a guy who best case is your 7th best player and is probably 8 or 9. He's a good defensive role player, but I don't think he has the versatility to be an every day impact defender. His offense is almost entirely catch and shoot corner 3s. He's a nice piece to have, but you can't tie yourself to paying him that money to be your 4th big.
He started almost all of last year. That's why Brogdon won 6th man.
My guess is their preferred 5 is White Brown Tatum Porzingis Rob. They've always kind of leaned double big with lesser (at least potential) combos than Rob and Porzingis. Rob can pass and screen on the perimeter and Porzingis can shoot. You might not prefer either on the perimeter against great playmakers, but they have enough athleticism that hunting a matchup too hard isn't worth it, especially with the length of the other to potentially lurk. They should work together fine. Tatum is competent if you want to go "small" and play him at the 4, but I don't think you want him to have the paint/rebound responsibilities when he's your offense's engine. He can play a lot of minutes but that's a lot for a season.
Edit: Brad also told us in the press conferences for the Smart trade that they viewed guard as a surplus and wanted another big. (Why I think Brogdon's ego is fine btw; their willingness to pivot to Smart means it was less about him and more about roster imbalance.)
Without arguing over your highly debatable first statement, my biggest issue with the Smart Porzingis trade is with what both players bring to the team.
Porzingis is mediocre on defense and on offense only adds one more player to the "give a scorer the ball and stand outside the 3point line" offense that the Celtics have been unsuccessfully running for years, and on top of they already have Tatum and Brown that are both better than Porzingis in ISOs, so all that he adds is a 3° option to a system that wasn't working with two great ones.
On the other hand no one will argue that Smart is not a good defender and you may say his decision making isn't that good on offense, and I agree with that, but in this last playoffs he was the Cs best playmaker, mostly due to the fact that most of the time he was the only one.
So with that trade they have basically worsened their defense in favor of having a even more static offense which already wasn't working for them.
I really don't think it's that debatable. Potential health and the difference in contracts are the reason they got picks out of the swap, not because they're equivalent players. There's a reason he's a perennial max-ish guy and Smart will never sniff it.
Porzingis is an efficient paint scorer, especially for your third option and with Rob as pure vertical threat. He's not just a catch and shoot guy on offense. He might not be your best scorer, but he's a damn good third option, and he's a great option against a zone team like Miami that wants to push you into taking short-mid range shots without getting to the rim.
Smart is a good defender, but he was far from a special one last year. He wasn't better at the point of attack or on the perimeter than White, and his standout trait defensively last year was that he wasn't a mismatch against bigs. Porzingis isn't a great defender, but he's also not a liability, and swapping him for Smart means you're even bigger while still not having anyone on the court that's easy to blow by. It also means Tatum can play 3 and doesn't have to battle in the paint as much to allow you to get rebounds.
Brad told you they had surplus at guard because they did. They want to play bigger and this lets them do it.