For instance, battleships died out due to very expensive ships being taken out by swarms of bombers, which were cheaper to replace and arm. The same principal is true for tanks vs RPGs/anti-tank weapons, but tanks are still used today.
The death of the tank has been pronounced fairly much since Egyptian infantry in 1973 started swatting the vaunted Israeli tanks . First it was ATGMs, then attack helicopters, and so on. However, to this date, nothing has yet been developed which can take the place of the tank on today's battlefield. Capable of engaging anything it can find, and being a threat to anything which can threaten it, it provides sustainable, protected mobile firepower. With the development of better protection systems, the balance has returned back to the favor of the tank. It still takes a ballsy, and somewhat lucky chap to nail a tank with an RPG, especially if the tank crew is paying attention and working with other arms like infantry for its own protection.
The flaw with the battleship analogy is that the ship's ability to project power and be the baddest thing afloat was completely usurped by the carrier which could do it all better. The infantryman with the RPG, however, though a threat to the tank, is not a threat to the tank's role which only a tank, so far, can do.