It's really sad to read how people feel about it. I loved the initial implementation of Investment Pool on launch, but also used the game option to bring it back only on one playthrough after it got changed. I don't cry, that it's gone, and agree with the DevDiary, that it really wasn't easy to support anymore, and that it would only get worse over time - it's good that they don't try to keep it an option now, arguably maybe it shouldn't have been a game option after the Private one was introduced, as it was always going to end being quite hard to have both systems possible.
But I also really respect that system. Coming from Vic2 where Capitalists would toss money at random buildings, that had no way of ever being profitable, and State Capitalism was so nice (at least for smaller countries), because you actually could decide to build something reasonable... Directly Controlled Investment pool was the best of both worlds. It let the player make the smart choices, but also you would build stuff that makes pops the most money, as it would mean more investment pool in the future. It had you build what you wanted, but also that aligned with what pops could have built on their own.
It's okay that the system ended up being changed, it's okay that it gets retired and won't be in the game anymore... but it's also good to remember, that it was a pretty damn well designed system. It saddens me quite a lot to see the OP treat it as obsolete or bad, or how people who preferred it are not seen as playing "the right way".
It reminds me of the FTL change for Stellaris, which on launch had you choose which of three differently working FTL systems you wanted to start with. It's not that it was bad, but rather for the future development of the game it needed to be reverted and changed.