You've used three terms for genres I've never heard before so I would classify them as niche at best and therefore discount them entirely. As for Grunge, was it ever really that popular outside the Northwest? Personally, I think Cobain did us all a big favor.
There was Blues, good or not, produced during the 40's but it didn't get much airplay due to "race music" restrictions by broadcasters. As for Rock in the 40's you would have to provide examples. I have never heard anything even remotely resembling modern rock during that decade.
There have always been genres of rock, and they all make up part of what rock music is. To talk about rock music, you're also talking about genres. Whether it was the Mersey beat stuff that was popular on radio in the early 1960's, surf music, mod, psychedelic rock, folk rock, progressive rock, or blues rock. They're all genres. Any of the bands you've mentioned could be placed in a genre.
I'm sure when you mention that 1955-1984 was the best period for rock music, you aren't referring to Ace's "How Long" (technically in the rock format) or Rex's "You Take My Breath Away" (technically in the rock format) or anything by Pablo Cruise (technically in the rock music format), or anything by bands like Ambrosia (not exactly hard hitting music, but still technically in the rock format). But those bands and songs were played on album rock stations during your favorite era.
They happen to belong to a genre of rock I never cared for. But they still were "rock", and still were very popular.
My point was that the genres I mentioned were examples of what made rock music valuable post-1984. Rather than list a bunch of bands you may or may not have heard, I decided to list the genres. There are bands in those genres that I thought made rock music that was as good as a lot of the rock music that was released during your own favorite era.
Grunge was popular world wide. Soundgarden, Nirvana, Pearl Jam and Alice In Chains all had albums that entered the Billboard 100 at #1 position in 1994. Much of the output by those four bands is quite good. A lot of it was very creative (especially Soundgarden). Nu-metal and rap-rock were popular world wide. Some of those artists sold millions of records, and a lot of those records were actually very good -- and some of it was also junk -- just like you get in any era of music.
As for the connection between rock and roll and blues in the 1940's, I think Avid covered that quite well.