It seems like a solution in search of a problem? How often do you need to embed a spreadsheet in word-processing document?

Furthermore, making components from multiple independent vendors interact seamlessly is just an incredibly complex problem. The web is probably the most successful decentralized system, and this is because the common integration point which all must support is very very simple: URL's and links.

Web components is more equivalent to GUI toolkits than office plugins. For web components to be a solution they'd need quite a lot of augmenting, like, a standard way to serialize and deserialize document-like content out of them. I don't think there is such a thing right now, having re-checked the MDN documentation on them, but if I'm wrong I'm sure the Internet will let me know.

You probably could augment your way up to something that would work more like OpenDoc without too much hassle in principle, but it'd be a lot of work in practice.