Earlier quoted context omitted.
It's a problem of most buzzwords, there is no formal definition and it largely depends on context. If I were to give you my attempt of an definition, it would be: the main idea behind "web3" is to have systems where no single entity can gate-keep users. It's not supposed to be "better than web 1.0", but mostly to be a contrast with the "walled garden" platforms and social networks that defined "web 2.0". So, by that…
Thanks; the last part seems like "non-repudiation" - I did not think we need either centralized entities OR blockchain for that; I may be wrong though?
You need to have distributed consensus to avoid double-spending. Can you have distributed consensus without blockchain? Yes, but only if your participants are selected a priori, e.g, Distributed Paxos.
The problem that blockchains solve is that it is the only practical way to solve consensus between parties that do not trust each other and that are not pre-agreed on.