Hi, it’s nice to see you thinking about old systems like the music theory, which originates from the ancient Greece!
I think the modes really have a function, because when you say: this song is in the Dorian mode, I know the song will sound peaceful (in my case, I assume everyone experiences the modes differently). In Phrygian mode, I know it will sound “Arabic” and intriguing. And so on.
On your example, these are two different things. In the VI - VII - I, the major I comes as a surprise, and it’s mostly used to end a song in minor (i) with the major chord, to make it finish in a happier way. This is called the picardian third (you’ll find some interesting Youtube videos on this).
In the bVI - bVII - I, the two first chords come as a surprise. The song seems to make a sudden switch to the minor key (i), and ends back in the major key. In the Pearl Jam song “Even Flow”, the chorus features bVI and bVII chords. So while in the first case, the I is the odd one, in the second case the bVI and bVII are abnormal and the I is the resolution.