Definitely understand the pespective. I think the major difference would be that Broward and Palm Beach's principal cities were already established with decent skylines of their own and economic base and never actual suburbs of Miami-Dade from a commuting perspective. They've all expanded since WW2, to the point of where they've grown into one large urban area with three major central cores. Throughout time, they've always been similarly scaled to what they are today:
1960
935,047 - Miami-Dade
333,946 - Broward
228,106 - Palm Beach
2019
2,716,940 - Miami-Dade
1,952,778 - Broward
1,496,770 - Palm Beach
On the other hand, we have a pretty big gulf. You nailed it with the scale observation:
1960
455,411 - Duval
30,034 - St. Johns
2019
957,755 - Duval
264,672 - St. Johns
St. Johns is pretty small. There are 24 counties in Florida that are larger and not in danger of being surpassed by St. Johns despite their growth percentage. For St. Johns to be comparable to Palm Beach's scale to Miami-Dade, St. Johns would need to double in population while Duval remains completely stagnant or go into decline during that growth period. The realistic long term scenario, if Florida fills in (hopefully we avoid this), St. Johns will likely become another sprawling low density Florida suburb (they'd be swallowed by Jax's sprawl) because they don't have any areas zoned for a dense urban cluster to incrementally develop.
Btw, looking at the numbers by decade, it is pretty crazy how fast Miami-Dade caught and passed Duval:
1900
4,955 - Miami-Dade
39,733 - Duval
1920
42,753 - Miami-Dade
113,540 - Duval
1940
267,739 - Miami-Dade
210,143 - Duval
1960
935,047 - Miami-Dade
455,411 - Duval