Won't this cause champs such as Tryn, Riven, Jax and the like to just be broken in 3's?

Is there a new item coming in as an effective counter instead?

I could be wrong but won't this just push the meta of 3 man buiser teams even further?

Riot cares not about your opinion or TT.

From what I can see, they want to make TT the new testing ground for concept weapons and objectives.

Bloodmail isn't comparible to Thornmail... it's flat HP with some regen for killing someone

Thornmail is a bad item

Get randiums omen/Frozen heart instead.

It's a trap in most cases, but in the niche case that it's useful, it's too strong. I also have a few design issues with it; for instance, it scales with your opponent's power (yes, I know there are abilities that do this), and it's targeted against a group of characters that is already weak on the map.

Thornmail seems pretty toxic in dominion in comparison to it's relatively fine/good uses elsewhere. But attack speed reduction is significantly better on summoner's rift though.
Randuins + Frozen Heart will always be more effective at shutting down Yi when any team fight post 30 minutes will have him at 0.8 attacks per second even with his ult (which has always been confusing about it making him immune to slows but not attack speed ones although he's immune to nasus' wither). Every new character having an auto attack reset makes thornmail a slightly better item than before.

Riven and Jax aren't countered by Thornmail. If you are building Thormail to counter them you are falling into a trap. Tryn is only quasi-countered by Thornmail.

Riven does a lot of damage through her abilities, not her auto-attacks (she does auto-attack but typically doesn't build much/any attack speed). The Armor will reduce her damage, but if you just want Armor there are many better items - Randuin's, Frozen Heart, Sunfire Cape, Atma's ect depending on your champs needs. Also Riven can build tanky and easily add Magic Resistance to her build (Maw of Mal, Force of Nature, ect.) which will make the Thornmail deal piddly damage to her.

Jax is much the same case, except a lot his damage is Magic damage and isn't even reduced by armor. Further, although he does typically build attack speed and attack fast, thornmail is only reflect a portion of his auto-attack damage as his Ult passive adds magic damage to his attacks. Again, Jax is naturally kind of tank and can easily add Magic Resistance to his build (on top of what he gets freely from his Ult) to take minimal damage form Thornmail.

Characters that are actually countered by Thornmail, such as Fiora, AD Yi and other squishy characters that almost entirely rely on physical damage from their auto-attack to do damage, already struggle on Twisted Treeline and it only benefits champion diversity to remove it.