I can agree it was the most linear to that point, but there were still elements of exploration, mainly from the big planet levels, the fact that there were secret stars, and that there were no time limits. Also the HUB planets that existed since Mario 64's paintings, where each level yielded 6-7 stars and you returned to explore a new part of the level.
There were some elements of nonlinearity in Galaxy, but you could probably say that about a lot of linear games. Honestly the Galaxy games are practically the definition of linear, and are examples of how great that kind of game can be.
It's divided up into levels, each time you enter a level you get 1 primary objective, with a mostly straightforward way of achieving that objective. Even though the stages have multiple stars, once you pick one you're stuck on it. If I remember correctly, the game will even alter the level to make sure you can't go too far off the beaten path of the star you've selected. If you choose to do one of the few secret stars in a level, once you complete it the level ends and you have to start over in order to complete the original objective.
Even powerups are handled in a restrictive way, sometimes adding time limits on them so you can only use them in the specific part of the levels you get them in.
The most nonlinear areas in the Galaxy games are the "collect 5 star shards or silver stars" and purple coins, but they're hardly the high point of the games imo.
In some ways I'd consider 3D World actually less linear than the Galaxy games. The levels may be smaller, but you're presented with more options. You can approach every level with a different power up (normal, cat, fire flower, or boomerang), a different character with their own movement style, single player or 2-4 players, and it had 4 collectables per level which is a slight step up over Galaxy. And despite the levels being small, they still have multiple routes and hidden secrets, including a couple alternate exits that allow you to skip ahead in the game.
Actually one of my favorite aspects of Galaxy was its linearity.
And I recall bringing that up as a quality of the game.
Yeah, Mario 64 put me to sleep honestly (though I played it long after release and maybe it just didn't age well). The Galaxy games kept me much more engaged.
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Back to NX, I really don't care at all about 720p vs 1080p, maybe because of the tiny monitor I use to play games on. I'd honestly be happier to have more 720p games if that means the horsepower is used on other things like framerate and effects.