If we want to discuss heartland then we have to focus on heart. If you rip the heart out of something it dies.
Therefore which countries would kill F1 if you ripped their participation out? UK and Italy.
If you removed the UK obviously it would mean the end of all teams apart from Sauber, STR and Ferrari who'd all lose some staff but could recover and continue with Ferrari, Renault and Honda engines. The logistics, management and TV companies would all need to be replaced.
Removing Italy would be the end of two teams and the need to swap to a different tyre supplier, and brake supplier, but the championship could mostly function with one team needing a new engine supplier.
If you tried it with France, you'd lose a driver and Renault engines, no teams, no GP. So a couple of teams would have to use other engines
If you tried it with Germany you'd lose a few drivers but no teams, no engine, no GP (this year)
If you tried it with the US you'd lose a GP and a team next year
Australia would cause the loss of a driver and venue, Brazil two drivers.
If you were to stuff UK into the memory chute, you'd be erasing a huge amount of F1 history (champions, teams etc), similarly with Ferrari , sorry Italy.
Obviously the above is very simplistic - you can always find ways around things. But I think if we are taking heartland we are talking about the UK, Every other country would in someway be manageable if it were to cease participating in F1, but if the UK stopped then F1's heart would be gone.
Although if you want to do stereotypes then Italy is the true heartland of F1, with the UK being the brainland.