First:
LOL
I'll make it simple.
As I said it is a nice alternative to the bahamut's staff, but saying it makes the Bahamut's obsolete just made you the laughing stock of the summoners.
But as you want so much to call gear obsolete because you aren't able to get it here's a simple comparison.
Elemental Siphon:
You get 2 more MP on your Siphons from the Mythic pole, which looks nice on the screen but you lose at least one time 3 MP because of perpetuation, setting you back 1 MP on your Siphon versus a Bahamut's.
It's 1MP but if you wish to take this discussion that way...
Bahamut's staff already wins vs your combo in this direction.
Avatar's Favor is not used on offensive playstyle, I do not know where you get that idea or where I even said that.
But this also shows that you know close to nothing about avatar's favor.
Allow me to explain:
Avatar's Favor does indeed reduce the avata's attack abilities, but according to those summoners that actually researched it the attack is reduced by a mere 15%, it doesn't completely cancel the summoner's DD abilities.
That said, if a summoner is in offensive mode, this should not be up, even to conserve MP, but any summoner can tell you that.
Now onto Avatar's Favor and the summoning magic skill effect on it.
The people I talked about that figured favor out also found out that the 'hard' cap for Avatar's favor buffs given to the party are reached when the summoner themselves get up to a summoning magic skill of 317.
To make it simple, that's a 4MP/Tick refresh.
A summoner's skill caps at 269 so to get to 317 you'd need +48 Skill in gear.
Let's add up in gear available to the average player that has no merits in Summoning magic skill:
Evoker's Ring: +10
Summoner's Bracers: +10
Summoner's Torque: +7
Summoning Earring: +3
Oracle's Braconi: +5
Evoker's Horn: +5
Nashira Crackows: +5
Astute Cape: +5
That totals +50 in gear, but leaves no room for perpetuation cost reducing gear except for your Crozier or Elemental staff.
Bahamut's staff already frees your one slot for another Perpetuation reducing item and does not sacrifice perpetuation cost.
Now up to your little rant:
Predator Claws 165 MP with an average damage of 550 on Tiamat.
Timed well I do 3 of those every landing session, which comes out to about 1650 damage at least every 4 minutes unless your blms blow her out of the sky every flight.
A merit pact is 182MP, the damage formula is as follows:
Formula is (floor[512+1.72*(TP+1)] + floor[(Avatar's INT - Monster's INT) * 1.5]) * MAB(of avatar)/MDB(of monster) * Day/Weather
Avatars at level 75 are estimated to have 82 INT.
Now I am roughly estimating Tiamat has 120 INT so putting the the calculation of you having a merit pact maxed.
That is the strength of a 160%TP pact for a fresh summon.
Calculation:
(floor[512+1.72*(160+1)] + floor[(82 - 120) * 1.5])
512+1.72*(160+1)
161*1.72 = 276.92 (Let's round this number up to 277)
512+277 = 789
INT82 - INT120 = -38
789-38= 751
751*1.5= 1126.5 (1127, which is about correct looking at my own Heavenly Strikes which I have fully merited.)
Now Tiamat has a very high magic defense bonus, some would say around 50% damage reduction when she's airborn.
That averages out around the same as physical pacts with the slight difference that if you have a little army of black mages nuking alongside you that you probably won't get enough time to get 3 of those off making you less effective in the damage department then doing your 'Lazy' ground attacks.
Also your MP vs. Damage ratio is less efficient as you use more MP for roughly the same damage.
(You guys don't sleep her on the ground do you?

)
Aside from that, Tiamat tends to resist a lot, even on the high int/elemental skill blms.
If you don't believe it, ask your own best geared blms.
Besides, it's not that I am completely Idle when she flies.
BLMs love Dream shroud, no matter how little it adds.
Also Elemental Siphon and Sublimation (when no rdm present) are awesome for MP management.
Physically Tiamat does not have a very high evasion, you have less chance missing a physical pact then getting resisted on a merit pact.
Besides that, physical pacts can crit where your merit pacts are stuck on their damage formula and the more then occasional resists.
Regarding killtimes, my linkshell does not do $3000 or 100+m gil for a piece of software which does not make you have to press enter every 29 minutes.
But we've been doing fine without, proven by the Riddill we've gotten last friday, the N-Legs the day before and the N-head the day after and the Gaiters on my own character's feet.
Disadvantage is that we do not get as many applicants as such linkshells so we don't have a lot of bodies to throw at things, or to even camp 2 or 3 things at the same time, also since my shell is oriented at European playtimes it happens that something can pop when less people are on.
Like the early afternoons, where most of us people work, or late at night when people are sleeping.
Most of us don't appreciate being called up at night just because Tiamat is about to pop.
But during those times when you're looking at one tank, a bard, a white mage, a red mage, a thief, a corsair and 3 black mages to take care of Timmeh as well as Earth and Air elementals and even our good friends the Arch corses at night.
A fight can get lengthy if noone else pops with a useful job.
But at least we do not wipe with more then an alliance.