I doubt very seriously that you will acknowledge it but DL took the MIA frequencies because it was available as part of the AA/BA JV divestiture, has managed to gain several flights using that strategy, and still has 2 flights/day on LHR-MIA via the VS JV.
MIA-LHR is a local market route for DL; ATL is a very successful gateway as a hub. It is the 2nd largest single airline gateway to Latin America behind AA at MIA.
when DL adds MIA-Latin America - which will very likely happen after DL digests the current round of capacity additions - it will be because MIA represents a strategically important market that DL needs to serve in Latin America.
Unlike AA's strategy in the pacific, DL started by expanding to Latin America from its strength hubs of ATL, JFK, and LAX and will add MIA based on that strength. AA is just now realizing that success at DFW to Asia is the only hope it has of making more competitive Asia markets work - but that for AA is still just concept and not reality.