Part of what is driving the valley's decline is its constrained size (both by geography and NIMBYism) coupled with its past success. The valley / bay area is now full of mega-corps that suck up most of the top talent and inflate wages and thus costs to levels startups and bootstrappers can't afford. There is no space to let off the pressure because its constrained by mountains, ocean, a fault line, and politics.
If the city were both able and willing/allowed to grow like New York, Chicago, or Tokyo did in their heyday of growth the engine might run a lot longer but as it stands the pressure is going to force the industry into diaspora.
Compare Shenzhen, new global HQ of the electronics industry. SV should have grown like that, but politics would have none of it. California liberals are anti-construction and California conservatives are anti-infrastructure.