The last 2 NBA champions

[Dahlsim]

Have both won by going "small," benching their starting center and protecting the rim with an elite PF.

Centers are still important for the regular season/certain matchups, but it's pretty clear where the NBA is moving. This is exactly why I'd go all-in on Collins... I don't think he'll ever be close to as good defensively as Giannis or AD, but he's at least the type of big man you can keep on the floor in the playoffs.

Centers are still important for the regular season/certain matchups

Actually the role of the centers and the ability to go BIG or small in the rotation was a huge factor in this playoff.
Brook Lopez was great in this series and without D'Andre Ayton the Suns don't make the finals.
I'm not sure how you define small ball.
Lopez a 7 footer played over 26 min AND Bobby Portis who is 6'10" played a critical role for the Bucks playoffs and those guys are on top of Giannis.
In fact by the end you could see Monty trying to figure out a way to match size up by bringing in Frank Kaminsky for some key minutes but the Bucks had the better big rotation talent.

In fact I don't see anything 'small' about Milwaukee's rotation except that are talented to enough to match up with every type of rotation.  They could go big and out-size your Ayton, who was a real problem for Sun's opponents but then they could as you say take the traditional 5 off the floor and play Giannis supported by guys like Porter and a big guard Jrue Holiday.

Giannis is an incredibly versatile big, who can play in the middle or handle the ball so he defies even using the word 'small ball' but even with that the Bucks were the bigger team here which allowed them to play BIG or small because they could force the rotation and match up as needed.
Giannis took the Suns inside or outside and could play on the floor at the same time as 7 foot Lopez for stretches.

Brook Lopez reminded us that he has been an NBA All star himself and had developed his game to score both outside and inside because he has always been an excellent inside and post up player.
The Bucks had pure talent and in fact all of Giannis/Middleton/Holiday and Lopez have been NBA All Stars.

That's 4 guys with All star type talent and none of them really 'small' for their position btw.

Small ball rotations have to be available to your team because you can't just dominate with the best center anymore. That much has changed. 
The team has to be able to match up big or small but if the opponents small ball team is better than yours talent wise then going small isn't an answer to anything.  You still have to be the better team and put the better talent on the floor because 29 other small ball teams went home losers. Two teams with Elite center talent were in the finals.

The centers don't dominate as in prior NBA generations but they are still easily and often the key difference between 1 small ball team winning and another losing.