I don't think that you fully understand the political environment in Portugal. To win a national election a party needs votes in the north and south. You cannot win a national election by ignoring or alienating either greater Porto or greater Lisbon. BA can ignore Manchester because they are not government owned.
I understand Portugal. There is nothing exceptional about it, as some of you want to picture it. There is nothing exceptional about OPO which at best is a secondary market. There are political elections and "regionalisms" in every other country in Europe. And in the upcoming crisis (which undoubtedly will make Portugal head towards a major economic crash), the last worry of most people in Northern Portugal is whether TAP has flights to Milan or Brussels from OPO.
Alitalia (nationalised) has largely ignored the wealthier, more populated North of Italy (which wants to become independent) to favour Rome. Iberia (even before privatisation) ignored Barcelona (and Catalonia) historically. And Milan and Barcelona are much more key and "controversial" markets than OPO, being Portugal a very "homogeneous" country.
Having this dual hub (LIS/OPO) was a sign of the times, of abundance. Let's say TAP had 100 flights, so 80 were to LIS and 20 to OPO. If tomorrow TAP downsizes to 70 flights... LIS is going to do everything in their hands to keep those 70 flights. Which is not also "natural" but also recommended in order to keep a competitive hub. Splitting those flights over two hubs (and neither TAP or Portugal are Lufthansa or Germany to handle two hubs like FRA or MUC) will only make things worse.