With how long it took Intel to ship expensive, incompatible, so-so performance ia64 chips - your theory needs an alternate universe where Intel has no competitors, ever, to take advantage of the obvious market opportunity.
It's true that most of those would have lacked the resources to replicate AMD's feat with AMD64. OTOH, AMD itself had to buy out NexGen to produce their K6. Without AMD and/or AMD64, there'd be plenty of larger players who might decide to fill the void.
With how long it took Intel to ship expensive, incompatible, so-so performance ia64 chips - your theory needs an alternate universe where Intel has no competitors, ever, to take advantage of the obvious market opportunity.