Why the Supermarine Seagull lost out to helicopters

Would the 1948 Supermarine Seagull have needed a folding tail to fit aboard catapult-equipped capital ships?

Probably not in principle, since it was designed around the Walrus's shipboard role; the larger reason it faded was the arrival of helicopters.

The speakers describe the Seagull as an enlarged development of the Supermarine Walrus, designed to fold into comparable shipboard space. Its intended work included reconnaissance, anti-submarine patrol, rescue, communications, and transporting senior officers when a capital ship operated independently. Carriers and task groups reduced the need for a dedicated capital-ship floatplane, while helicopters increasingly performed the same missions more flexibly. The Seagull therefore represents a highly developed piston aircraft retained as insurance while navies tested whether the new rotary-wing technology would prove dependable.