PSA: Don't buy Merc Treads on tanks
I mean a tank as in somebody who will generally build 6 items dedicated to taking less damage and has innate tankiness, but this holds true for any champion that wants tanky boots whose primary method of staying alive isn't by using mobility.
I'm going to use the pre-S5 change costs since I feel it puts it into perspective better. The total cost has not changed at all.
Before the change, it was 400 for the mantle and 475 for the upgrade. You went from 20 MR to 25 MR. As you can see, this is completely shit for defensive itemization and will never really make a massive difference.
Comparing this to Tabi, which reduces all AAs by 10% in addition to 25 armor at a lower cost, you are getting a lot more bang for your buck since you are all but guaranteed to have at least one enemy that will feed into the 10% reduction of the boots. Considering that the champions who would be affected most by the removal of Tenacity on Juggernaut are all tanks, these are all champions that are better off with the 10% reduction. Even on bruisers like Jax/Irelia, you are probably better off with Tabi unless they have some stupid comp that has Veigar/Fiddles/Rammus and they all like CC'ing you.
tl;dr merc treads suck for tankiness and you should get tabi unless you're something like Kass/Zed who need the CC reduction to stay ahead of their opponents and survive