Every musical generation suffered from sounding the same

I disagree.

You can't claim to be VoiceofReason and drop something like this.

All music has defining characteristics. Take symphonic music, for example. There are certain qualities that identify a composition as belonging to the Baroque, Classical, or Romantic periods. The same applies to rock music. Each decade has a certain sound that has become associated with its era. Here's some generic observations about the decades of the 20th Century:
• 1950s - The birth of rock-and-roll. If you listen carefully, many of the songs have a simple 3-chord progression taken from blues music. Groups were small, consisting of guitar, bass and drums. Saxophones and pianos were a common addition to this core of instruments.
• 1960s - The early part of the decade was an extension of the 1950s until the British Invasion and psychedelic era. The music became more layered with the addition of horn and string sections.
• 1970s - Psychedelia evolved into progressive or art rock. The genre continued to split into different sounds like hard rock, Southern rock, pop rock, etc.
• 1980s - More sub-genres like "hair bands," Metal and synthesized pop dominated the playlists.
• 1990s - The synthesized sound died out as more bands returned to traditional acoustic and electric instruments. Alternative rock became prevalent.

As I said, these are only generic descriptions. But I think you can see what I'm driving at.

Yes, I can hardly tell the difference between the music of Guns N’ Roses and Rick Astley.

You're asking for good lyrics in rock groups, and rock groups rarely had good lyrics. Lyrics were secondary. You want good lyrics, search among singer-songwriters. Check Leonard Cohen, for example.

Each genre has its own interests, and most of them don't care about lyrics. You check classical music, and you'll find that lyrics are extremely forgettable. Classical composers literally didn't give a shit about lyrics. The only classical genre that really cares about lyrics is Opera.

It depends, I think there used to be a lot more "diversity" in genres than there is now. Look at the 1970's for example, Led Zep, Black Sabbath, Deep Purple, Kiss, Jethro Tull, AC/DC to name a few they all sound very different. Same with the Beatles and the Stones very different.

From about the late 90's onward though you start getting a sameness. Nickleback, Daughtry, Creed, Matchbox 20, Good Charlotte, 3 Doors Down etc Same with pop music modern R&B or whatever they call it now all sounds the same, rap is all the same. So many of the songs are written by the same people too which doesn't help.

I completely disagree... There was SO many kinds of different music in the 50-70s.. Not only rhythms, but melodies, chord structures.. Rock, Jazz, Classical, Blues, Folk, Latin, Reggae, Eastern influence, bossa nova, Ska, R&B, Big Band Bebop, Samba, Country, etc...... and also the production (which is usually disregarded).

If you're not interested in a music it's going to sound the same. This is nothing new. Most people lose interest in finding new music at a point relatively early in their lives. This is nothing new.

And the same ramblings about music come out from every generation. yadda. yadda. yadda.