It is interesting to me that it is Jrue Holiday who is at some level carrying the expectation to "get the C's an NBA title". He is a new player so I get it but why not Porzingis?
The way I am trying to look at it is what was missing from last season's team that prevented us from winning the title. What needs to be added or fixed to make the team better? And whatever that was, can Jrue Holiday and/or Porzingis address the need? The problem is that I am not sure that what went wrong against MIA had anything to do with personnel.
Last season, Rob Williams only started 20 regular season games and 4 playoff games. He was not a starter. If the assumption is that the Celtics will be primarily a 1-big team, like last season, Porzingis becomes the 1 primary big and I see that as an upgrade over Horford. Horford becomes the primary back up. Porzingis + Horford seems like an improvement over Horford + Williams.
As to combo guards, last season, due to the smaller line up approach, the guards were Smart + White + Brogdon. Now, if you assume a similar positional rotation, the combo guards will be Holiday + White + Pritchard. I see Holiday as an upgrade on Smart, Smart was good, Holiday is just better. But now we have Pritchard getting Brogdon's minutes. That is not an upgrade. So is this guard rotation an upgrade? Probably not if Pritchard plays as much as Brogdon did.
Our wings are about the same, Tatum + Brown + Hauser, any improvement here will have to come from individual development.
The last grouping is what I consider big depth. Last season it was Grant + Griffin + Kornet. In my mind, Grant has been replaced with Brissett and Griffin at this point is being replaced with Wenyen Gabriel. That gives us Brissett + Gabriel + Kornet. It is hard to gauge how important this grouping is. During the regular season, Horford and Porzingis will miss games so this group is going to see minutes. Not so much in the playoffs. Brissett and Gabriel may turn out to be important new pieces.
So the net of all this is that from a purely personnel standpoint, I think we are only slightly better. Holiday is important but so aren't Porzingis, Brissett, and Gabriel (or whoever ends up in that role). I think there were some buy in issues last season due to the abrupt coaching change right at the start of the season that ended up rearing its head in the MIA series. Hopefully the team is over all of that.