Thanks for the reply. After I little bit of digging, I found that it's much clearer why the changes to Trinity Force were made. The changes weren't targeted at Trinity Force at all, it just inherited the stat changes from the component items. Phage had its AD lowered by 5 because it's a bruiser item, not an ADC item. Zeal had its crit chance doubled from 10% to 20% and its attack speed lowered to make it more of an ADC item. Sheen had its AP removed and its mana increased and got CDR, making it significantly better on bruisers who wanted to build Sheen but not Lichbane.
The overall changes might have made Trinity Force slightly less appealing to bruisers, but that doesn't mean that Riot had any intention of making Trinity Force the ADC Sheen item, so trying to call them out on that doesn't make any sense. Crit might not be great for Bruisers, but they've always paid for crit when building Trinity Force. And for the AD/AP/AS that bruisers want, they now get Guinsoo's, which is ridiculously strong. I don't think all of the Trinity Force users I listed were getting it, but I know at least Irelia and Jax love it.
Trinity Force is about more than raw stats, which is one reason why it's always going to be more of a bruiser item than a traditional marksman item. The two passives are fantastic on most bruisers. Bonus movement speed when you're hitting something means you can't get kited as easily, and Spellblade works great with the high base AD of bruisers.
CDR is okay on every champ, but it's significantly better on champions with a lot of strong abilities. Traditional marksmen who are autoattack reliant might like CDR, but it's way down on their stat priority list because most of their damage is coming from autoattacks, which are modified by AD, Attack Speed, and crits. Bruisers, on the other hand, tend to love CDR because it allows them to gapclose more, CC more, and put out more damage.
As for who is building Trinity Force, champion.gg lists it in the most common and highest winrate builds of all of the champs I listed. Hecarim top and jungle both use Trinity Force more often than IBG and win more often with Trinity Force than IBG. If you have a source for good irelias or jaxes saying Trinity Force isn't worth building anymore, that would be cool.
For the Black Cleaver, the cost increase is partially due to the increased value of the stats now that it builds out of Caulfield's instead of Kindlegem, and partially is a nerf targeted at juggernauts. Deadman's Plate has been receiving a bunch of nerfs for the same reason: juggernauts are supposed to be low mobility, low cc, high damage, and high health. With the Black Cleaver and Deadman's Plate, they were still low on cc, but now had high damage, high health, and high mobility. The Black Cleaver still has all of the things that juggernauts and bruisers want though: shred, cdr, mobility, and hp. The Black Cleaver is still being built on juggernauts like Garen, Illaoi, and Darius.
Sort of tangential to your last point, the Hydras probably won't be free to switch anymore after the next patch.