Are you implying that the downfall of Detroit was global competition?

OP wouldn't be wrong to do so. Detroit (the city) suffered through the changes of suburbanization and racial tensions during the 50s/60s...but Detroit (the industry) also suffered a steady decline in automotive market share starting around then as well. This had equally devastating effects to the region. Just look at Flint.

I believe Detroit automakers used to account for 90% (I may be off by +/- 10%) of global car sales coming out of WW2.

By the 2000s Japan, Germany, and others had eaten away half of that share in the US market, and an even higher percentage in global markets.

Silicon Valley has had a many decades head start in Tech just as Detroit did in Automotive (let's say 1900s-1960s for autos and 1970s-2020s for Tech). To think global competition won't eventually start eating away at this dominance is foolish, especially given the profits at stake.

What’s less clear to me is whether or not the tech industry is like the car. Will the rate of innovation drop as we slowly approach an asymptotically perfect product, or will it accelerate as access to concentrated labor and knowledge drives up the rate of change?

an excellent question.

however, it may be orthogonal to the question of whether or not other parts of the US, and more broadly, other parts of the world, can be where that continuing development happens. regardless of which of those two paths turns out to be the future, there's still the question of what SV's role will be.

Depends how strong the network effect is, I guess.