You hit the nail with this one.
Having Fox-Domas as your two best players are always going to cause issues because they both struggle to shoot the 3 ball AND defend. I want to talk about both of them together because their limitations go hand in hand when it comes to roster construction and style of play.
Regular season issues:
Focusing on the just the offense, their lack of shooting means that we need floor spacers around them. Everyone in the starting lineup has to be at minimum, a good shooter. Both players are ball dominant and play best with the ball in their hands whether it means facilitating or scoring. This means that everyone around them should be able to move without the ball. Monte did a great job last offseason surrounding Fox and Sabonis with both of the above, and we ended up with the best offense in the NBA this year: Prolific 3pt shooting, prolific scoring, great movement, good passing, etc. Barnes was a solid 3rd option because he can consistently create his own shot and score in isolation. The offense was not a problem for us.
Focusing on the defense, we have major issues on this end because of both Fox and Sabonis. Fox struggles to contain the point of attack while Sabonis struggles to protect the rim. It's a recipe for disaster that leads to a chain reaction of bad defense. Fox's man blows by him > Sabonis' lack of length struggles to alter shots > Kevin sinks down to help > Barnes rotates over to cover > Someone doesn't rotate to Barnes' man > wide open 3. Repeat re-rinse for the regular season. In general, poor defensive PG play is not unexpected and is easier to cover, however, poor interior defense is more uncommon and harder to cover. This means we need a legitimate interior defender because if they get pass Fox or Huerter, they'll still be met with force/length down low. But we've already decided that Sabonis at the 4 didn't entirely work to the best in Indiana. Sabonis is also not the most lateral player and struggles to contain the perimeter. The Kings avoided switching Sabonis as much as they could this year. Defense was a problem all-year long and it's how we let team stay with us.
Playoff Issues:
The Kings' entire "woes" in the playoffs were much different than our regular season struggles. It flip-flopped, and our offense became the problem, while our defense was solid. This was a very matchup dependent struggle. A large part was due to the Warriors being a 3pt reliant team who facilitated off of Draymond and Looney. They don't have consistent down-hill attackers, which didn't lead to a lot of open 3s unless it came off screening action and DHOs. What actually killed us on defense was our lack of rebounding. Not enough boxing out, not enough high-pointing rebounds, and just not enough effort. This created a lot of second chance opportunities for the Warriors. On offense, our 3pt shooting completely went cold for reasons unknown (maybe the moment was too big for Huerter). When our 3pt shooters aren't making their shots, it forces Sabonis, Fox, and Barnes to create their own offense. Sabonis struggled badly against Looney and Draymond. I don't want to break this down into exhaustion, but Sabonis was a non-threat from outside, and also got stonewalled in the paint. Fox was the one who really responded to the task... but a 1 man show can't win it on his own. Barnes' just wasn't consistent enough during the series as a scorer.
Moving Forward:
I think you're right that we will most likely not reach the same level of 3pt shooting we saw this past season. I think this team has a lot of issues and I'm not really sure how we can fix them or where to even start. Any defensive improvements to our starting lineup will very likely lead to a less offensive-powered team. We've won the entire regular season off the backs of our offense, but can we still win teams on a more balanced roster? It would help the team a lot of Sabonis could shoot 3s.. but he's in his 7th year and showed no type of rhythm from deep. He doesn't even look to take the shot most of the time. To fix Fox and Sabonis, I think this team needs a PF who can shoot 3s, rebound, and protect the rim.. but the list of candidates are unattainable. I think this team also needs a legitimate 2nd/3rd option scorer. Fox went cold during the 7th game, and the entire team wilted. In all of the regular season, our gameplay in the 4th quarter was to give the ball to Fox and ask him to work magic... but what if magic doesn't work? I think we've need a guy who can help alleviate some of that shot-creating. We can't really solve all the problems, but Monte will just have to pick and choose which ones he'd like to address first. There was always going to be short-comings with the Fox-Sabonis duo, but for a team who just went to the playoffs for the first time in 17 years, I think you just have to keep rolling with it. I'm also not getting concerned about Sabonis unless he repeats this type of playoff performance again next year.