I think some important things are left out in this discussion - things that are needed to assess the issue of Led Zeppelin plagiarising music fairly. Led Zeppelin, in their early days, were not a studio band. They were a live band. What they played live differed significantly, and even dramatically from what they put out on their albums. Dazed and Confused, six and a half minutes long on record, was extended to a musical extravaganza that would be between 10 and 30 minutes long at their gigs. Same with Whole Lotta Love. Moby Dick during their live gigs was essentially a 20-minute drum solo. They weren't songs anymore, they were sonic experiences. And what's more important, no two performances were the same. Sometimes they would just play whatever they felt like. And this is why Led Zep became so famous - they were bloody good at it. They would turn a gig into a jam session and remain so tight and focused that audiences were captivated by it.
However, since they were so focused on playing live, in the early days, they didn't put much effort into composing. They knew that whatever they put on record would just be a starting point for what they would do later onstage, and they recorded and released their early albums to promote their gigs. Their first two albums were written and recorded under immense time pressure, and Led Zeppelin II in fact was recorded while the band was on tour in the US. So yeah, they ripped off a lot of artists, and at least in some cases probably even did so knowingly. And some of their original writing efforts weren't particularly meaningful either. Robert Plant, for instance, is known to have simply improvised the lyrics to some songs during recording, and that's why they make no sense. He just sang whatever popped into his head, and one of their songs is literally called Ramble On. But they didn't care back then, and neither did their audience. And they probably never thought this would have consequences, because they didn't look that far into the future.
Led Zeppelin III was their first record on which they actually devoted quality time to writing, and they gradually began to put more ambition and effort into their compositions. But I think it's not justified to judge their artistic quality as a band by their studio albums - as odd as it admittedly sounds retrospectively and with our present day attitude.
To a lesser extent, what I said also goes for Deep Purple. Many of their studio albums sound very flat, but they released their records so that people would go and witness the tense energy created by Blackmore, Lord and Gillan. People didn't go to the gigs to check off Smoke on the Water and Highway Star on their "heard live" list, but to experience what these people did onstage.