This is a limit more than an advantage, IMHO, because it imposed the standard structure verse-chorus-verse-chorus-bridge-chorus-chorus that, at last, became prevedible and boring. 70's compositions were longer, freer, less standardized, more imaginative.
Yes, but thats not really a sign of a good rock song. Neither one of the qualities, nor all of them in combination. Stairway to Heaven is nearly a too long song (And still largely follows the standard structure), and quite many Deep Purple songs had been a disastrous ego war between Blackmore and Lord.
I don't think that progressive rock really deserves being called progressive. Like it had been said: "Perfection is not reached, when there is nothing left to be added, but when there is nothing left to be left away".
You could complain of course about boring song structures - but I am not sure, if that is that simple, because without a clear structure, a song does also not work. Like a good book or movie, a song also needs a clear structure and pace to work. There are only few longer songs IMHO, that have been written with a clear concept in mind. And the best longer songs are IMHO not written in the 1970s - take for example Gamma Rays "Rebellion in Dreamland", which is also faster paced than 70s rock songs. On the other hand, you have Dream Theaters "Pull me under", which is the longest justification for an awesome drum intro.
Also, what has to be said about long rock songs had been said with Manowars "Achilles, Agony and Ecstasy in Eight Parts".
But is generally accepted that metal take origins from early Led Zeppelin and Black Sabbath works.
Yes, origins. But then, it travelled a LONG distance from that. Even Rainbow, which was still in the 1970s, had only little in common what Led Zeppelin or Black Sabbath called rock. And Motörhead was rather citing what was before Led Zeppelin and Sabbath. Also Krautrock was also a phenomena in the 1970s, that was nearly unknown in Germany and rediscovered in the 2010s.
I would ask a question. Here in Italy, the perception of the German music market is that Rock has more consideration there than in other countries. It is true?
Can't really estimate this as German native. We have a pretty large music scene in general, and it is pretty diverse as well. We have traditionally a large metal scene here, we have a strong rock music tradition in Germany (Ton Steine Scherben as early example).
But I am not sure, if that is special or just because of the scale of the German music market.
In Italy, not only metal (that is "at home" in Germany) but the rock as a whole is a little overshadowed in the present day. You cited Bon Jovi and is a perfect example: here in Italy that group has very, very little consideration (even in his "golden years") and is somewhat derided, but in Germany has ever a larger success, even with frankly mediocre albums as the last three or four.
Well, we are largely americanized so its no surprise that the US 1980s left some dent in western Germany - and that did also not stop in the east.