Why opposed landings survive by avoiding the defender's strongest point

Are opposed amphibious landings obsolete, and what replaces them if they are?

No, but a sensible assault avoids the defender's main concentration, lands where limited resistance can be overwhelmed, and builds combat power ashore before striking the primary objective.

An amphibious force should not prove its courage by attacking the most heavily fortified point available. At Normandy the Allies chose a weaker stretch of the Atlantic Wall that they could support and sustain, while at San Carlos the British avoided the Argentine concentration around Stanley. Aircraft, artillery, and local troops can still oppose a landing, but the attacker seeks a place where naval and air support can suppress those defenses long enough to establish a lodgment. Helicopters, hovercraft, and modern landing systems expand the usable coastline and make it harder for a defender to concentrate everywhere.