I think you need a big forward who can anchor your team in a lot of ways to win in today's league. But it doesn't need to be a MVP level guy. Draymond does the job just fine for GSW, LBJ did it for the Cavs/Heat, and Kawhi/Duncan did it for the Spurs.
As the league has gotten smaller so has the size required from this sort of player. It could be a true C like Davis/Embiid but it no longer HAS to be. Not anymore.
One of the reasons I was optimistic about the C's making the Finals before this season went to crap is that Horford fits that mold of a big man who can survive in the modern NBA and hold everything together. Sadly everyone around him turned into a bunch of wet drama noodles.
I agree.
Though I would say that I think talking about "needing" a player with a specific skillset at a certain position is too reductive, especially in the modern "positionless" NBA.
I think the reason that big men were always so valuable is that having a great big man provided a consistent, reliable way to get high percentage scoring opportunities (i.e. in the paint), and also gave a team an edge in terms of grabbing rebounds and preventing the other team from getting good looks.
If you look at constructing a contender as an exercise in putting the right elements in place to increase your margin for error / lower the degree of difficulty for scoring more points than the opponent, having a great big man, historically, was the easiest way to get there.
As you point out, these days a lot of teams accomplish it with a combination of elite outside shooting and multi-talented forwards.
I think one of the biggest problem with how this year's Celts team was constructed was that they simply didn't have a reliable, consistent option for maintaining their scoring. All of their most talented players stood out primarily by creating jumpshots. The offense only really looked dangerous when the lead guys got really hot from outside. They didn't get to the line, they didn't create a lot of transition opportunities, they didn't get many second chance opportunities, and they didn't regularly attack the rim. Really tough to win that way.
That's part of why simply bringing back Kyrie and Horford along with Hayward, Tatum, and Brown seems like a futile exercise. Even if the chemistry is better, they're still going to be a dish with too much of one flavor.
I can imagine a team built around a great guard and a balanced supporting cast, but that guard would need to be able to create a lot of high percentage looks for himself and others (contrast with Kyrie who specializes in hitting an above average rate on low percentage looks).
Houston is a team that comes to mind in that regard, but the Warriors have been able to solve them because they play the Rockets' big men off the floor and defend with discipline so Harden can't get to the line a million times.
To some extent though, I wonder if there's any point in thinking about what could theoretically beat the Warriors when considering the best way to build a contender. Is anybody beating the Warriors if they dont' first beat themselves? Even in 2016 the Warriors had to make some major unforced errors for the Cavs to have a chance.
In a league without a crazy juggernaut on par with the Warriors (i.e. the league for most of the period between, say, 2002 and 2016), a team like the Rockets probably would have come much closer to winning a title.
Chances are the next Celtics title will come after this iteration of the Warriors is either done altogether or is distinctively on the decline.