While you do have a very long and well-thought out argument (I'll really give you that), you seem to be underplaying how much of a disadvantage having a main unit that can't take cities is.
For one, the air transport lowers a bomber stack's range by 2, so it's not AS bad as you think it is unless you have pure bombers going out to attack something.
At a cost of 350 (minimum cost, this is with the -200 from SM and -50 as an SP upgrade) the air transport is incredibly expensive. While 350 may not in reality seem like very much, it is coupled with a main offensive unit that is 130, and is practically the only way to take a city with a bomber stack unless you happen to have an infantry close to the city you're going after. So, the only way to take a city is to have that expensive unit flying around, and at turn one it means that you're going to have to buy one of them per city you expect to take. If you want a good opening, you're typically going to have to buy around 3-4 of them, which is over 1k down the drain JUST to get to the cities. Buying the offensive units to TAKE the cities is going to cost extra, and at an early level this digs into funds extremely quickly and only leaves certain countries open to even be feasible for an SM starting spot.
My third point is also that air transports are practically the only way to take a city. While it sounds like i'm being redundant, I do have a different point. I stated earlier that a good SM opening is 3 or 4 cities, which means you have to buy 3 or 4 air transports. The next turn, you'll have 3 or 4 air transports, so you can also take 4 cities, provided you have the bombers for it. At this point, you won't have money for more air transports, so you have to do what you can with what you have. As the turns drag on around 3-7, taking 4 cities (max) per turn is getting pretty tiresome. You either have to devote your entire stack to take a city with 1 or 2 militia, or don't take it at all. There are much more important choices that need to be made in this regard when it comes to SM, as you want to take the largest cities first, but you also want to get a country bonus for that little extra boost of income. You can't just buy an air transport and bombers to take that tiny little city, it'l cost a fortune, where normally you could buy a tank or two, or even divert two tanks from one of your larger stacks and take two cities with one original stack. This also becomes a huge problem when attacking somebody. SM may seem like its large range makes it extremely useful while attacking, the range is making up for the fact that you can only take one city per stack. For instance, if you were attacking Ukraine, with normal units (marines, tanks, even infantry) you could take the cap of Kiev, but also several of the outlying cities if you have a stack large enough. With SM, only taking Kiev is possible unless you're insane and brought two air transports with you, which may be a viable strategy once to break up a stack you predict having to be broken up, but if used repeatedly would quickly bankrupt you. While this may not seem important, your opponent could place 50 tanks in one of the cities, knowing that you can only take one city. Take out the tanks, or take the cap and risk having your bomber stack destroyed the next turn? (in this case, I'd take out the tanks, but if it were a more reasonable amount of tanks that would be a tough choice) With tanks, you could send 8 tanks to Kiev to take the cap and send the rest to the reinforced city and win both battles. I also would like to point out that you can't 'swarm' with bombers, as you can with so many other strategies, for the same reason as I've been going on about. If you see massive walls of tanks coming at you, it's going to be hard to defend everywhere, but if you see a huge bomber stack coming, you can tell where it's going and prepare that one spot against the bomber wave.
My final reason is going back to the cost of the air transports, costing anywhere between 35-60 per turn to keep them in the air. This requires people using SM to think several turns ahead of where each stack is going (obviously this is normally required of any strategy, but it's needed on a larger level in this case) because if the logistics are bad, and air transport could end up taking a city where it would take several turns to get back into action to take another neutral (or hostile player) city. If done with normal units, it's not a large drain, and getting those few back to where they needed to be won't be a big deal, but it is if you have to get back an air transport AND the bombers back. It also requires a larger amount of skill to play the strategy and isn't 'mindless bomber spamming at the enemy' like so many think it is. Where's the single best place to attack, that will be both hard for the enemy to reinforce, good to take, and also practical to take.
I'm not trying to say SM's the hardest strategy out there and should never be used. It's a great strategy and the incredible range of your stacks makes it amazing if you aren't a retard and know how to properly attack with air stacks. However, it comes at the price of it being costly, with a much slower opening than many comperable strategies, and has more skill requirements than the average strat. Also your 'balances' are way too OP in the first place, 25 is ridiculous, they could make a fortress with that in Europe if America was sending bombers, and having to buy a new air transport for each attack is bullshit.