I'm not familiar with Santa Monica airport, but I am extremely familiar with the three Toronto area airports which have closed since I started flying, and a fourth which is on the verge of closing. In all cases the closure was in large part to a shift in the balance of public pressure and increasing land values, to the interest and revenue generated by the airport. The shift was always away from aviation - our industry simply does not create enough revenue to justify the investment in prime real estate for an airport.
General aviation is at best tolerated by the public, but as often disliked and feared. They live their entire lives believing that they have no interest nor benefit derived from GA, so they will make little concession for it. So too, nearly all politicians.
The Toronto airport next to go will be Buttonville, late this year. I have been a regular user of this airport for 40 years. It existed as early as the '30's at that location, as my mother tells of taking flying lessons there in the late '40's. During the most recent 40 years, I have seen this airport go from being "one its own" surrounded by fields, to now being surrounded by built up area. There is nowhere practical to force land near by. There have been several airplane vs building and airplane vs road traffic crashes in recent years. I now fly high and close circuits there, just in case...
This very evening I dined with a pilot owner friend, who has been a Buttonville tenant for 30 years. He is now faced with relocating his 'plane a further 30 minute drive from where he lives, as the next nearest suitable airport. There simply is no other choice. Equally to his dismay, he reports that the very active Buttonville Flying Club will lose it's meeting place, as the next most suitable airport lacks facilities for that.
There is no solution for city dwellers to assure convenient GA facilities, the two are not harmonious. Where they co exist, pilots should think themselves fortunate, and go out of the way to appease the local non pilot citizens to delay the inevitable as long as possible...
I miss these old and convenient airports, as I miss Toronto International CYYZ welcoming GA... yes, I used to fly a 172 and 310 based there. I used to fly that 310 to Meigs Field in Chicago, I miss it too. Anything we can do to preserve these airports is great, but our options are limited.