Let me first say that I am positive that 5E will allow you to play either way, but what should the default be?
Do you like and think we should hold onto a targeted end for a campaign? 30 and out, although it might be 20 or 40 in 5E. Personally I like the planned end. Without it, some campaigns might never wrap things up and there is something satisfying about saying, "we finally did it." The alternative is that the campaign will break down before the story is done because the story is never done and the campaign will break down eventually.
It's hard to say, because both ways have advantages and disadvantages, both in terms of mechanics and in the assumptions that go along with them.
3E had unlimited leveling, and 1E did as well provided that you were of the right race/class combination. This let you set your own end for when a campaign "should" stop (that is, you never had to cease playing because you hit the hard cap on levels). But as you leveled more, there were increasing assumptions about the relative power level in the campaign world. If you could reach level 40 in five years of in-game time, why weren't there level 100 mentors around? Why was a level 50 encounter considered a world-wide threat, etc.?
2E, 4E, and BECM (notwithstanding the I) had a hard cap on the levels you could gain (though 2E's wasn't stated in the PHB, but came later on). This allowed for a clear end-point to the campaign, and let you design the game world based on a clear universal designation of power (the Immortals set did this too, but simply stacked on top of fixed levels). However, it also meant that you didn't have the freedom to explore themes that couldn't be adequately addressed in the set structure of the game (e.g. you're just fighting
bigger demons at level 30, but not an undead
planet, since that's beyond what level 30 is built to handle). Likewise, it shut you down if you wanted to keep the game going past the limit.
Level caps, and their implications on world design and epic-level gaming, are like the rest of the game in that they don't have a "right answer." There are different play styles, and different gamers prefer different ones.