My dad went to Roosevelt just prior to WW2. I don't remember him telling me about any wall or MECHA. Things must have changed since then.
It doesn't take much to "change" an area. Illegals dump a bunch of anchor babies on the local school system for babysitting services (assisted by a sh*tload of charities) and the American natives no longer want to send their children to that school system to learn English as a second language. So-o-o, they either homeschool, revert to private schools, register their children at an address outside that district, or move out of the newly formed barrio. POOF! Insta-Slum.