Agreed to some extent. Except the 101 Freeway wasn't the northern boundary of downtown even when it was built. There were always parts of downtown north of the 101, which partially explains why the southeastern approach has the 101's control city as the LA Civic Center but doesn't treat the 10 as being the approach for "south" downtown.
When the 101, San Bernardino Freeway, the Harbor Freeway, and the southeastern loop (Golden State Freeway and Santa Monica Skyway) were planned and built, they definitely weren't seen as defining the borders of downtown. Bunker Hill and what would become the Financial District were undesirable neighborhoods filled with homes. Downtown at that time was a corridor from Olvera Street down Main and Broadway to about Olympic. It was narrow and tall.
While some of it eventually filled out to some of the freeways, other parts didn't. The Arts District and the Warehouse District east of the LA River are very difficult to describe as being part of a downtown. Everything south of the Financial District (e.g., South Park) are only very recent additions to the urban core while rest south of there (Old downtown and the area around the Metropolitan Courthouse) were urban almost even before the freeways.
Arguably, before the freeways, LA had an hourglass shaped downtown, thick at the north end near Olvera Street, Union Station, and the govt buildings, thinning out until Wilshire, and then thickening out through the theater district all the way down to almost the Adams District. While time and deliberate gentrification projects have pushed some of these boundaries outward and fattened them a bit, the boundaries of downtown being defined by the freeways is true only to the extent that one could define the entire city of LA as being defined by the boundaries of other freeways.
I leave you with this. Here's the north end of the Figueroa corridor next to the 110 just south of the 101. Today this is chock-a-block with high rises. But in the late 70's, even with the massive push in the 50's and 60's to gentrify nearby Bunker Hill, most of it is a parking lot or empty lots. This is one of the corners of the supposed downtown defined by freeways, and even 40 years ago--long after the freeways that supposedly defined it had been built--it was still surprisingly undeveloped.
