You still see Mercury's Treads more than any other tenacity item. This was never really because it offered the most tenacity, but because it offered Magic Resist and level-2 boots all in one item.

Mercury's Treads
Recipe: Boots of Speed (350g) + Null-Magic Mantle (400g) + 450g = 1200g
Stats: +25 magic resistance, +35 tenacity and UNIQUE Enhanced Movement 2

Cloak and Dagger
Recipe: Cloak of Agility (830g) + Dagger (420g) + 200g = 1450g
Stats: +20% attack speed, +20% critical strike change and +40 tenacity

Eleisa's Miracle
Recipe: Philosopher's Stone (800g) + 500g = 1300g
Stats: +25 health regeneration, +20 mana regeneration and +40 tenacity

Moonflair Spellblade
Recipe: Blasting Wand (860g) + 340g = 1200g
Stats: +50 ability power and +40 tenacity

(Alternate) Moonflair Spellblade
Recipe: Haunting Guise (1485g) + 340g = 1825
Stats: +30 ability power, +220 health, +35 tenacity and UNIQUE +25 Magic Penetration

Increasing the tenacity at least poses alternatives. And a suggestion, whilst I'm at it;

Dragon Heartstone
Recipe: Heart of Gold (825g) + Cloth Armor (300g) + 250g = 1375g
Stats: +350 health, +20 armor and +30 tenacity
Passive UNIQUE: Grants 20 increased tenacity for 5 seconds after the application of a crowd control effect. Stacks 3 times. Increased tenacity will not affect active crowd control effects.

I can't see increasing the Tenacity to 40 making people use the non-mercuries unless in very special situations. Hell, make it 50 and they'd still be niche. They lack the same value as Mercuries and one DPS/Caster/Tank/Support item, and about 80% of the time Crowd Control is on Magic Damage. Having bonus MR just helps even more, and then putting it on the inevitable item slot for boots makes it a lot better than a different pair of boots and a Tenacity item.

Also, since tenacity doesnt stack its best on boots so you dont waste one of your core items to get some. Thought that last item sounds nice i would rather have my heart of gold build into a randuins if i make one.

I feel that making Tenacity a Unique Passive on most of the items severely ruins your chance of survival. No one would honestly stack all Tenacity items because you really wouldn't have any end result that would really be positive, but even still. I think if you really needed to, it'd be nice to have 3 items with tenacity. In all honesty, sometimes when you have a team in 3s that is all slowing champs, it'd be kinda nice.

Fair idea. Perhaps lower the tenacity you get from items, and make it unique per item rather than global-unique. I'd definitely get Mercury Treads + Cloak and Dagger on a lot of peeps. I trialled getting Berserker Greaves and Cloak and Dagger, but I learnt the hard way that it was the MR that I used more than the tenacity.

Too much cc reduction when you get over 35%.

You'd definitely have to have it stack multiplicatively (as dodge does), at most, but I could see it being possible.

The only time I have used it is with the Miracle on a support champ. The CDR boots are nice with the philo stone for g/5 and sustained mana/health early and make it for tenacity later. As said above, they really aren't exactly gold efficient and scale pretty bad into lategame, they don't replace merc treads/item. Maby if they were really good early game items people would use them.

The idea behind giving all items equal tenacity, is you can now buy a cheap, efficient item that will boost your stats as well as give you tenacity, and free up your option in boots.

The items providing tenacity now are just fine. Moonflair and Cloak and Dagger provide bonuses so that any champion will benefit alot from either one, and Eleisa's Miracle is good for agressive gameplay.

Can you imagine Trundle with even more tenacity? Or Irelia? Theyre already next to impossible to stop when they have their buffs up.