I find the Civ V cultural victory to be in every way superior to Civ IV. Culture in Civ IV was a half-implemented feature. It did not meaningfully improve your empire, the only reason to really focus on it was if you were going for a culture win. Think about the UBs like the Pavillion, really no benefit whatsoever to the non-cultural player. In Civ V a culture win is a culmination of a long effort to improve your empire through social policy. I actually think Civ IV culture win was MUCH more linear. To win culture, you have to basically shut down your empire from normal tech/progress and put everything into culture, which resulted in a long grind of accumulating purple music notes. In V if you want a well-functioning empire you will be paying some attention to culture, whether or not you're going for that vic condition. Anyone in a commanding position in the game save the all out super-expansionist should have a cultural vic open to them.
I agree that diplomatic vic in V could use some work. But to say its more linear is odd. The main complaints, with which I strongly agree, are that you can play the whole game having nothing to do with CS, then suddenly get a bunch of allies with accumulated gold and win. That gold could also be conceivably deployed into spaceship factories, or GDRs if the player made a different choice at a very late stage. So most people complain, rightly so, that the diplo vic is not linear ENOUGH.
A simple change would make CS vote for the player that had accumulated the most total influence with them over the course of the whole game. So someone with a longstanding relationship with the CS, doing quests, gifting gold since ancient times, etc, gets the vote over the recent FOTM suitor with a large modern bankbook. This would be much better, IMO. But it rewards the longstanding CS devotee, over someone who is changing playstyles and victory goals on the fly. More linear then?