> What does "portable" mean?
In the context of the web, portability means that you can, ideally at least, use any browser on any platform to access any website. Of course that isn't always possible, as you say. But adding a big new restriction, "these websites only run on x86" was very unpopular in the web ecosystem - we should at least aim to increase portability, not reduce it.
> And was NPAPI not a part of the web, and a key part of its early success? Was ActiveX not a part of the web? I think they both were.
Historically, yes, and Flash as well. But the web ecosystem moved away from those things for a reason. They brought not only portability issues but also security risks.
Why should we aim to increase portability? There's a lot of unstated ideological assumptions underlying that goal, which not everyone shares. Large parts of the industry don't agree with the goal of portability or even explicitly reject it, which is one reason why so much software isn't distributed as web apps.
Security is similar. It sounds good, but is always in tension with other goals. In reality the web doesn't have a goal of ever increasing security. If it was, then they'd take features out, not keep adding new stuff. WebGPU expands the attack surface dramatically despite all the work done on Dawn and other sandboxing tech. It's optional, hardly any web pages need it. Security isn't the primary goal of the web, so it gets added anyway.
This is what I mean by saying it was vague and unclear. Portability and security are abstract qualities. Demanding them means sacrificing other things, usually innovation and progress. But the sort of people who make portability a red line never discuss that side of the equation.