A thousand generations of people have raised their children in cities without cars. We haven't evolved as a species in the last 100 years.
Your argument seems to balance on this fulcrum, which ignores several more-salient realities:
1) Those thousands of generations didn't have "destination" Museums and Aquariums, either
2) Leisure time and family time are recent inventions (1880 - 1950), with "quality time" being a 1980s concept.
3) We have not evolved physically, but consumer expectations are light-years from where they were 100 years ago.
4) Corporate donors donate based on "viewership"/visitorship. Kill the visits, and the sponsors leave too.
5) The New England Aquarium, almost by definition, needs to draw visits from rich, upper-middle class people who've scattered themselves all over New England to sustain itself.
6) This is not a "is it possible to make a trip to NEAQ" the question is "is it PREFERRED" to make such a trip as a leisure consumer.
Even in Boston, 85% commute by car* so, yeah, most consumer preference for walking as part of "leisure fun" ends at the most distant spot at the mall.
MFA? Own Garage, Surface Lots, On-street, Transit.
MOS? Own Garage + Transit
Lego? Free Garage + Transit
Coco Keys, Laser Tag, Rock Climbing, Malls, IMAX. Drive, drive, drive.
If you're cheap, how about a nice 2K or 4K video of fish at home? Or, if you're rich, SeaWorld Orlando opened in 1973 (a decade after NEAQ) and can out-fish, out-experience, out-touch NEAQ for that one day every year or three your (rich) family is "into fish" (and they have better "water" weather 80% of the time).
The Aquarium has to *win* the contest to get a family's 4 hours and $100 to $125, (or $150 to $200/yr). Second place is first loser.
Make the Aquarium the first loser for 85% of families, and you'll kill it. Kill visitorship, kill sponsorship, kill the whole thing.
This is not just a Boston problem. On bad weather weekends, a newer wing of The American Museum of Natural History on NYC's Upper West Side (a destination made possible the the streetcar, originally, not walking, and made prosperous by the subway), has more minivans on 2 underground levels, parked closer together than any factory lot. Ask even them if they could live without their garage.
*I suspect that 95% of "moms and dads" commute by car, because it turns into the ballet/karate/gymnastics/soccer/basketball/track shuttle. Suburban kids** walk nowhere, and changing their lives isn't going to start with the Aquarium.
**except mine, and a sprinkle of free-range parenting devotees, but that's
enough to support a branch library, not a waterfront aquarium.
After being members of NEAQ in 2004 (Nemo mania) and 2012/13 (when our visits were summer Sundays parked on street in the FiDi), we joined the
Virginia Aquarium in 2014 and went every rainy day (4 days?) during our Virginia Beach vacation. For us, the Aquarium is now officially a beach vacation thing, not a transit thing because, even from my Zone 1A home in Medford it is still a 3-seat ride (Bus/CR-GL/OL-BL) with an awful trip back (hard to time leaving the NEAQ versus catching CR trains that only run every 2hrs weekends, or even vs the 80 bus that only runs hourly, and that's assuming no melt-downs/aching feet).