It's always been age on marathon day that counts, not age on qualifying race day. As long as your time was set in the qualifying period and you'll be 50 before the marathon in April 2017 you're fine for GFA.
Much of what you (barley) say is true. There's definitely a law of diminishing returns though - once you've got to the point of being able to complete an IM course without technique falling to pieces and are confident enough that the IM swim melee doesn't bother you, then you're starting to need a lot more swimming to take off minutes.
I could spend more time swimming, but the reason I don't isn't because I think I'll get better gains on the bike, it's because I just don't enjoy swimming all that much. Triathlon is a hobby I'm doing in my spare time, so I'm not going to spend more time than I have to doing something I get no pleasure from. Open water isn't so bad, so now that the lakes are opening my swim time will go up, but ploughing up and down an indoor pool through the winter is done just to give a reasonable base.