That's kind of irrelevant to the adoption potential of WebGPU.
Those examples you gave are not comparable at all, ativex and flash are way, way higher level, don't operate at all like graphics API middleware.
PNaCL was a WASM alternative design, which thankfully lost as WASM is much more flexible.
My point is that WebGPU is way better positioned than any middleware, it has industry backing and official support from all relevant platforms (or plans for it).
It's also benefitted a huge amount from hindsight.
Additionally wgpu and WebGPU aren't the same thing, as wgpu exposes native features as well.