I'm not sure where you'd want a hard cap to fall, but it would likely be very bad for the Celtics regardless of what number it was set at, so no, I would not be in favor of a hard cap (in addition to the one that already exists in some scenarios).
It has been fun watching Belichick and the Patriots outgenius everybody else in the NFL, but the current Cs would be screwed if this were to happen. We currently have three max contract guys and are developing three high draft picks on rookie scale contracts (with another still to come). Under a hard cap, the Cs would either have to plan on dumping Irving, Hayward, and Horford OR trade Tatum, Brown, and Smart at some point.
I think because of how small the rosters are and how important just a couple of guys can be to a team in the NBA, it might not work the same as it does in the NFL where you sign your big contract guys and then fill in the dozens of other roster spots with players that fit your system. Superteams are a problem, but mostly because of the cap jump and the 2nd best player in the NBA taking much less than he actually should be making. The former will work out itself, while the latter could get even worse with better endorsement opportunities for the top players.
I would actually be in favour of this idea I think, even if it does hurt the Celtics short term I feel it is the only way to achieve genuine parity in the league.
The fact is, the way the NBA runs right now, it is built to allow the strong teams to get stronger, while the weak teams keep getting weaker. Teams are allowed to go over the cap in order to sign vet-min guys, so elite teams like Cleveland, Golden State and San Antonio keep getting more and more elite because they always have a never-ending stream of veterans who are willing to take minimum contracts just for the chance to try and cop a free ride to a championship. Meanwhile weaker teams in small markets can't sign free agents even if they want to, because the vast majority of quality players have zero interest in gong there.
Look at what the Cavs did to get to where they are. They extended Kyrie, signed Lebron, then extended Kevin Love to a massive deal. At the time those three guys took up pretty much their entire cap, but they used good old bird rights to throw $10M at Shumpert, $16M at Tristan Thompson (etc) and however many $m extra at other random vets to fill out that roster - and they got a championship for it.
If you have a hard cap in place, suddenly a team like the Cavs needs to think long and hard about their decisions. Maybe they re-sign Kyrie, Sign Lebron - have to think twice about signing Love, knowing they wouldn't have enough pieces to fill out the roster. Or maybe they do sign love, keep Shumpert, but have to let Tristan Thompson walk because they don't have the cap space. Maybe when they have 3 or 4 extra vets offering their vet-min services, they can only take one of those guys instead of 3 of them.
Now suddenly all of those extra guys (be it Love, Thompson, or those extra vet min players) can't sign with Cleveland - so it puts them on the market for other teams to pursue, and hence you end up "spreading the wealth" between teams, so to speak.
Now you can no longer have 3 or 4 SuperTeams who have loaded rosters because their filthy rich owners don't think twice about playing luxury tax. If players want to, for whatever reason, join together and make a superteam - then they would need to take MASSIVE pay cuts in order to do it. And by MASSIVE I mean like Kyrie taking $10M instead of $20M, or Lebron taking $19M/Yr instead of $30M/Yr - that scale of massive. And if a player wants to win that badly that he's willing to sacrifice $50M over 5 years to do it, then good luck to him - many (most, even) will not be willing to do that.
And for Boston, I don't think it honestly does hurt us. We would likely have to choose two out of our three max contacts to move forward with, and cut the third. Based on our youth movement, I would guess that would mean keeping Kyrie and Hayward, dropping/trading Horford to somebody for a lesser paid big man who can still produce.
But you know why that is ok? Because we wouldn't be the ONLY team who'd have to cut guys.
Cleveland are currently paying $85.4M to Lebron James, Kevin Love, Isaiah Thomas, Tristan Thompson and Jae Crowder. With the Cap at $99M, that leaves just under $14M left for them to fill out their roster. That means they HAVE to cut JR Smith ($13.7M) because he'd take up their entire bench cap on his own. That leaves them with Shumpert ($10.3M), Kyle Korver ($7M) and Channing Frye ($7.4M) still taking up$24.7M, so they can probably realistically only keep one of those three guys. I'm thinking if you had to choose, you keep Korver - so that means Shumpert and Fry are gone. Now you have $92.4M in salaries being paid to only 6 players (Lebron, Love, Thomas, Thompson, Crowder. Korver) - leaving you with $7.6M left to fill out the rest of your roster. We add Derrick Rose ($1.5M), that puts us at $93.9M total. You probably keep Jeff Green ($1.5M) over Richard Jefferson ($2.5M) for his youth and flexibility - so that puts you at $95.4M total. Add Zizic ($1.6M) as their only real backup center and you're at $97m all up.
So now Clevleand's roster is:
Isaiah Thomas
Jae Crowder
Lebron James
Kevin Love
Tristan Thompson
Derrick Rose
Kyle Korver
Jeff Green
Ante Zizic
With $2M left to add one more vet-min bench guy. Strong starting five, but that bench (other then Rose) will get punished big time. And they would have no option to re-sign Thomas after the season, unless Lebron and IT agree to both sign for like $15M apiece - highly unlikely.
Similar deal with the Warriors, who have $97.1M currently allocated to their starting 5 of Curry/Thompson/Durant/Green/Pachulia. That leaves $2m for a bench - not going to happen. They're not cutting Curry or Durant, so they'd need to scrap Thompson or Green...and they'd also need to cut Iggy ($14.8M). That's the bare minimum before they could start working on assembling a team.. And Warriors are great still, but once you remove Thompson/Green and Iggy the go from "unbeatable" to "exceptional, but beatable".
For Boston we'd be ok by Comparison. Our bi three of Hayward/Kyrie/Horford only take up $75M..so we could keep Tatum and Brown and still have $14M left or so to fill out our roster. We'd be able to compete with Clevleand and GS due to our depth.
Everything bcomes more even, more fair.