Right I know all of that theoretical stuff, but my question is in the real world; what actual things will actually work on Milotic? No one is going to bring in their Intimidate Landorus against a bulky water anyway. You can't really switch Milo into Defog Zapdos. Aegislash doesn't work at all. Low Sweep usually comes from Breloom, which you again cannot actually switch Milotic in on.
Defiant Bisharp works because he can realistically pull a few of those tricks, since he traps Aegislash w/ Pursuit, and immunity to Intimidate works as a bonus even if you never get intimidated (since he's a physical attacker). Milotic misses out on both those perks. You can name a full list of things that CAN trigger Competitive, but how many of those things will actually happen? More importantly, what can you do to increase the chances of them happening (and exploiting them successfully)?
I forgot about Shadow Ball/Moonblast/Psychic though. That's kind of a fun bonus. Gives you a chance of punishing Sylveon and GengarThat makes me think running a specially bulky Milo is probably the best way to go. At least that way you can switch in on Lati@s, Sylveon, Gardy, etc... AND actually beat them one on one.
I just ran some calcs... with 252 SpAtk EVs, and a neutral nature: +2 Milotic 2hko's Aegislash with Surf (even if it stays in Shield form). It 2hko's Mandibuzz, Pinsir... It 2hko's Greninja no matter what type it is (and even if it stays a water type). It 2hko's Tyranitar, even in Sand.
(all of these are even after leftovers recovery).
So if you can actually get this thing a boost, there's no question you could do some serious damage. And none of the above Pokemon can ohko Milotic, either. And Hydro Pump obviously does even more damage, but I'm not a fan of Hydro Miss.
If I were building a team with Milo, I'd make it Calm, 252 HP, 252 SpAtk, lefties, and I'd put it on a team built over Spikes, and Stealth Rock (and maybe Toxic Spikes too). That way you can continuously punish Defog users, and if they don't have Defog, you still win.
I personally don't think Milotic should ever be roaming the OU tier without any physical defense evs and a Bold nature because her base defense is really quite low. Without defense investment, Milotic can't survive many of the OU hard hitters' attacks. That is why I would advocate the standard physically defensive spread for it in OU.
The aim of using Competitive on Milotic is more of a deterrent effect against opposing defogs and to instantly transform Milotic into a tank. Why use surf and hydro miss when you have scald, an awesome water move with a good burn rate that is invaluable? The point is, Milotic really doesn't need to sweep; to me, it serves a different purpose compared to Bisharp and their typings are dramatically different so it really depends on what you want to fit on your team. You want hyper offense? Run Bisharp. You want bulky offense with a strong FWG core? Run Milotic.
Even without sp def investment, gengar without tbolt is not a threat. Bulky zapdos with zero ev in sp atk can't even 2hko Milotic and if you're lucky in predicting a defog and get that boost, Milotic can KO zapdos or at least deal super heavy damage. Obviously, Milotic should never come in on Breloom. And I don't agree with you that Breloom with low sweep is very popular. Most brelooms still run the standard bullet seed and mach punch set. Besides, Bisharp dies to Breloom anyway if you really want to compare.
In all, you trade a little offensive power for better durability in order to survive in the current OU metagame which to me, is worth it. As mentioned in earlier posts, there is no point investing too much in sp atk such that it compromises on bulk because whatever can stop Milotic will stop it regardless i.e. chansey, blissey, sp defensive rotom-w. Is aegislash really a threat to Milotic? I think not because scald can easily destroy aegislash, especially with the added burn rate. The same goes for Gyarados and Mawile (they have intimidates and some of them actually think they can come in on Milotic with impunity); with defense evs, Milotic is now able to handle them (just play smart with Mawile since it is always slower than Milotic and play rough will never ohko Milotic. Gyarados is deathly afraid of a burn but avoid those with substitute as it gets troublesome. Then again, Milotic is not a hard counter against Gyara unless it runs HP electric which no one does anymore. Milotic is at best, a second check to those gyarados without substitute). And greninja is even more of a joke to Milotic because even with life orb and 0 sp def evs for Milotic, Milotic is at best 3HKO by its dark pulse (again, play smart!). Only those with grass knot are a threat to Milotic. A Milotic at good health is certainly able to keep greninja in check and burn it to death. A competitive boost to a Milotic with 0 sp atk evs can still deal heavy damage to greninja using just scald.
So my point really is, it is just not true that Milotic can't pull those tricks like what you have asserted. It all boils down to your strategy at the end of the day. Comparing Milotic with Bisharp is just moot because we have to acknowledge that the two are played differently even if they serve the same role as defog deterrents. We all agree that Bisharp has priority along with its excellent base attack stat of 125 but it doesn't mean Milotic is useless. Play to Milotic's strengths and I guarantee that it would not disappoint.
EDIT: So with defense evs, you can now deal with tyranitar and obviously if you had gotten a boost before that then tyranitar can be easily blown away by Milotic. My preferred set is still 252 HP/ 252 Def/ 4 Sp. Atk, Bold nature with scald, ice beam, haze/toxic/dragon pulse/dragon tail/ and recover.