The dungeons being optional doesn't mean they're still not a core component of the experience. BotW and TotK are fundamentally more similar to A Link to the Past or Ocarina of Time than to Link's Crossbow Training or Tri Force Heroes. Yeah, you no longer have a key item, and instead you're just...
Honestly, I've barely played Cadence of Hyrule because the rhythm based gameplay was a huge turnoff for me so no comment on that game.
But BotW and TotK definitely use the dungeon structure even if it's in a different way than past entries with most dungeons being optional.
To me mainline games are more about the structure of progressing through dungeons in an overworld until you eventually reach the end destination and fight the final boss.
I think Echoes of Wisdom will utilize this format so it's a mainline game to me.
I agree that the multiplayer games are a...