Since the Mahan's times world armies have been mechanised, new branches of the military have come into existence (aviation, nukes, etc.) All these have changed the warfare drastically. This idea makes me sceptic about the Mahan's works' importance now, while, frankly speaking, I'm not aware of his works.
would draw unwarranted conclusions from their works is no criticism either.
I meant his idea on the Eastern Europe:
"Who rules East Europe commands the Heartland; Who rules the heartland commands the World Island; Who rules the World Island commands the World."
Did Germany command the Eastern Europe in WWII? Yes, until they lost the war. They also commanded all the the "Heartland" except for Britain.
Did the USSR command the Eastern Europe? Yes, for 45 years. Did it make the USSR master of the world? No.
The "Heartland" Mackinder describes is not Europe but the Eurasian heartland - Central Asia broadly considered including western China and Siberia - the castle keep of the world, so to speak.
The Soviets were under no threat of invasion and conquest from any direction after the crushing of Germany as a power.
As regards air forces and nuclear weapons having displaced naval power as balancing factors to the perennial Land Power, I will say that the strategic space is now shared between Navy and Air Force, in what proportion I do not know.
I would only say that Mahan's Sea Power books definitely opened my eyes to the evolution of sea warfare from the early 17th C. to his own time.