I literally just played through DMC1 last week and the bosses do not change in any significant way to change the fight.
Every fight still required the exact same strategy. The only thing that changed was the backdrop.
You also forgot that you fight Nightmare three times as well with again, no significant change.
It's just nostalgia making people thing an older flawed game was worse than a newer game because the newer game is "different".
The amount of lies in a single post is almost overwhelming to deal with.
The 3 griffon fights are vastly different due to the location forcing to behave differently and the way Griffon himself fights (Stays mostly airborn in fight 1, keeps on harassing Dante with the red bird in fight 2, stays on the ground spamming new electric attacks endlessly in fight 3) Only a moron would consider the 3 fights identical.
And it works for the others: Nelo Angelo 3 has a completely different pattern (ghost swords) compared to fight 1 and 2, Nightmare 3 has several new attacks and his rebirth mode at the end, and Mundus fights aren't even remotely similar to each other.
The only one who stays pretty much the same with more attacks is Phantom.
Next time you spread some lies around, try to use some common sense first.
Like one of the above users stated. It was like different phases only instead of being back to back it was spread out to different fights. Same could be said of Vergil in DMC3. I still prefer DMC3s bosses but DMC1 had it second best. --- When coincidence is so convenient, I prefer to call it fate.