There's no such thing as true dynamic content unless it's actually run bypeople. All the automated dynamic content fucking sucks - see Rift.
It depends. If you refer to "monthly content changes" ala Asheron's Call, then it requires people to make, but not to run. People liked that, just having snow extending from the mountains and winter beasts coming down the plains in winter was a breath of fresh air that is still remembered 16 years later. And that no one made again.
If you want responsive dynamic content (i.e. stuff that changes in response to player actions), then you get into two problems:
1) It requires static servers; the concept of fixed servers is probably a dying concept in the MMO space since it leads to dead and empty servers very quickly, and causes lots of problems if you really are a MMO when people find no one around them. A fairly large number of MMO are starting with "megaservers" immediately rather than getting to implement one later. However, if you want dynamic responsive content and your zone vanishes after 2 hours because it"s now empty, then it becomes silly.
2) If you want interesting dynamic content, as opposed to scripted content (which is what Rift implemented, and what you have in a few of the GW2's zones), then you need to run the risk of failure. That is, you have to run the risk that the players can screw up. And devs are deathly afraid of having people finding themselves in an "unconfortable" situation.
Example: there's this mid-level zone, let's call it Kitticar Grove, and a nest of Undeads got seeded by the devs. And for whatever reason, on a handful of servers, players neglected it, and when the server woke up, the entire zone is a massive death trap that nobody in the proper level range can survive. And if the devs intervene, it makes a mockery of the responsive dynamic content idea.
(this is the reverse of the problem that AC1 ran into when one server didn't "cooperate" with their content story and defended to the death the Crystal 24/24. Since they didn't had truly dynamic content, they needed to cheat which was a slightly frustrating experience for all those involved).