Will be interesting. Not sure it will deliver on "Will keep more fans interested for longer."
There'll only be 3 extra finals' games, presumably played in a week. Two teams will be out after that, so those fans are only engaged for an extra week after the regular season ends.
And the top 6 teams were decided by Round 13 of the 2021-22 season. All that changed between R13 and R21 was where in the top 6 each of those teams finished. The only way you'll keep more fans engaged is if the contest for the last couple of playoff positions is tight. Which is more likely to happen when teams 5/6 both have losing records or break-even records (go back and check the NBL history and you'll see what I mean). Fortunately, that will be most seasons in a 10-team competition.
2021-22 was unusual, in that teams 7-10 were so bad they only won 16% of their combined games against the rest of the league. So teams 5 and 6 collected 19 of their 31 wins against those bottom 4 teams. Complementing that, they ended up with 16 of their 25 losses against the top 4 teams. Hence they both ended up with winning records.
It's likely that future 10-team seasons will see 5th placed teams finish 13-15 as often as they finish 15-13 and rarely finish 16-12. The 6th placed team will usually finish with a losing record. For that not to be the case would mean that a large competitiveness imbalance would have to exist (like this year). i.e., bottom few teams are way worse or top few teams are way better than the rest. Ongoing competitiveness imbalance is not good/engaging for fans of most teams. Think the period of dominance by NZ and Perth during the 2010s.
So this is not about fixing any problem with winning teams missing out on play-offs, as that will not happen often. It'll create a situation with losing teams consistently making play-offs. But that's fine, as I don't think fans of those teams will mind.
The only real down-side is for (fans of) 3rd/4th teams, as they now have to play an extra game or two before semis, and they now have to win at least one of those games to make the semis. i.e., they have to achieve more to make the real play-offs than they did previously, and they have a small increased risk of losing a key player to injury, in getting there.