See, to me, I disagree and agree. I like linear, as long as each linear place isn't the same. When galaxies started using the same planets but with different shades of colors, or at night, or similar things like that, it was draining. I had been there already, I had played it, just with different light. I enjoyed the linearity of each star. What I didn't enjoy was that so many of those ships were all nearly the same thing. Several bosses were repeated, with little, if any changes made. That's not an issue with linearity or exploration, that's an issue with design. Rather than calling a night-time version of a galaxy a new galaxy, if you're really going to re-use it, then just do what SM64 did, and make it a star collection difference between the different appearances.
The truth is, if you know your objective, the games aren't all that different. In SM64, you can do things out of order, but you really move in a line to the star. In Galaxy, the difference is that more unnecessary pathways are closed off, where in SM64, they wouldn't be, and you'd be left to figure out where to go on your own. For me, I feel that over-complicates things, makes finding your way confusing. The game is about discovery, yes, and experiencing amazing places, but I think that a large map can make things difficult to find the way, that it wasn't clear enough in some missions. The issue is that Galaxy, while fixing that, regurgitated things far too many times.
Then, at the end, you unlock a new character, and are told to do the entire game over, again, with Luigi. At that point, you've cleared the game, a long game, at that. What's the point of playing it again with Luigi, other than new control feeling and one more galaxy?