You know better than this. They didn't lose value without replacing it. They had max contracts guys walk out the door and replaced with other max contract guys. They went from three-ish All-Stars to 2 All-Stars.
Haybird and Irving were replaced by All-Stars JT and JB. Rookie years JB and JT were replaced by GW, RL, and AN. Frankly, everything the Cs have done personnel-wise with the exception of having Kyrie fall into their laps was more or less pre-ordained by the salary cap once it became clear that JB was a max or near-max player and JT was a max player. Yeah there was some tinkering they could have done but at the end of the day, we all knew the Cs were going to go as far as JB/JT would lead them.
We'll all wonder what could have been in 2017-18. I remember the super excited posts about pre-season games where the Cs looked like they could win 70 games. It sucks that it didn't work out but if GH hadn't gone up for that alley-oop, I doubt anyone would have been complaining about picking up Kyrie.
THis is a terrible way to look at this, and anyone who did this as a GM would be fired. The most important thing in the NBA is to lose as little value as possible. If you have rising stars who will need to get paid the max... you need to get value for your max guys. Even the cap thing isn't really accurate, the Celtics are under the tax, while paying $45M to Horford/Richardson/Hernangomez, they would much rather have better younger players in that $45M, and they'd like to trade some of the rest of what they have for a 3rd star... problem is they have little to trade in part because they step by step downgraded Horford/Kyrie/Hayward and a mid-1st to Horford on a bad contract and a trade exception, some of it was yes bad luck, but it doesn't change that this team would look very different if they hadn't gotten negative value out of those moves. Even just having a couple extra picks would be huge. And not being able to compete meant using another pick to dump a small salary.
Thinking of Tatum and Brown who were on the roster as the replacements for Kyrie/Horford/Hayward is IDIOTIC. They may have replaced some of the salary (though they make less of the cap since they are on 1st post-rookie contracts not 2nd or 3rd), but the TALENT wasn't replaced. This team was set up to add a bunch of talent to the Jays, instead they didn't add any talent they weren't already in line for. We are constantly talking about how hard it is to find a 3rd star, and how we'd get outbid... we'd get outbid because we burned a ton of assets to get and then lose top talent.
Edit- I should note, not all of this is the GMs fault, but it still happened. They chose not to trade Hayward because:
1. They probably overestimated their title chances
2. They thought they could re-sign him.
They were wrong on both.
They thought they could re-sign Al... they couldn't.
Kyrie... they let him walk to sign Kemba... okay, that one there was a plan... it ended up terribly though (bad luck? or perhaps they mis-evalutated how much time his knee had left.
The thing is though... those things still happened, they still got no real value out of having 3 very attractive assets.
In 4 years when both are reaching their peak? I'm not saying they should punt until then. But Jordan and Isiah were both 27 when they won their first title. Tatum is 23. Patience.
That's a problem.... neither guy is under contract that long, and keeping them both (or even one) gets tough if you burn 3 years not really competing.