"Linear" and "Open world" are two extremes, too. There's a happy medium somewhere in between that the other games used to get right and was completely lost with X. Even XIII-2 was structured completely linearly but they rather sensibly decided to branch out options from there to give you some form of choice.
But it's also a problem if people have such expectations from every new Final Fantasy game. I quickly learned that there's no common feature between the games in the series and so it would be wrong of me to expect the next game to have this or that or to be one way or the other. Every new Final Fantasy is different from its predecessor and it always gets some heat for it. But it shouldn't really if you condition your expectations.
And to me FFX never felt that linear. I remember playing Blitzball every now and then going back to previous locations to find sidequest items or hidden summons.