I can't really discuss the later part of your post, since numbers are numbers and for every early success theres likely someone who burned twice the gil. Even if the %chances didnt drop to 1/4th of the first meld at the second meld, randomness will happen. But if theres up to 10 less of a stat per materia, then a double meld is basiclly what is currently a single meld (assuming the best materia is +10 instead of +20).
As far as the first point, while that may be all you "need" for the current hardest fights. Thats certainly unlikely to get you into say, Garuda for some LS'es. And certainly not likely via PUG. The AF most people are going pass on is the ones that there is no real quality replacement for. (DRG - Boots for example) Experiences differ, this may be one of those times.
If 5.3 mil was the price to obtain your triple. Perhaps look at the price for the double, and look at the amount of slots you need to double. Then As the X amount of Jobs you'd be asked/needed to take. If you have a static where you are X job only, thats great. If not, you need to be whats needed at the time. Which is even more double melds.
The issue i think is for the amount of stat gain that can theoreticlly be had (+100 stat) And the most likely found max (+60) and any amount among that needs to be taken into account when they make a Bossfight. If the Theoretical Max was (+50 Stat and the likely was more around (+30 or +40 since odds are slightly higher) then i think stat gap, and the amount of better outfitted Jobs the entire playerbase could bring would be in a better spot.
Not to mention with a tighter spread of stats, Even a skilled group who may lack the resources (or luck) for better melds could still pull out a win by the skin of their teeth. Since the High point for top end melds would be a little closer to the AF "Average"
My opinion is a single meld with +9 or +10 Should be a repectable bonus, but up to +20 should be risk. Less risk then 24%, but +20 in a stat should equal some risk.