Quote Originally Posted by Ibi View Post
You don't have to, but you run into problems if you don't.

Under vertical progression, the developer knows that the items a player is getting within any tier will be made obsolete in six months. As a result, they can set a drop rate such that a player will, on average, get all (or almost all) the gear they need within that six month period, because there will be something new coming in by the time they're done.

Under horizontal progression, the developer knows that the items a player is getting from any particular event could remain relevant for several years. If they set the drop rates the same as they were in the vertical example above, then their player base is done with that particular event in that same six month time frame. So far that's not, in and of itself, a problem.

In both cases, the developer is adding new content at around the six month mark. But where the vertical progression developer doesn't need to pay attention to the old rewards when creating their new event, the horizontal progression developer needs a way to add drops that serve some sort of purpose, while not making the old items obsolete.
Just to counter that, you can have a mix of both and have horizontal progression within each tier. The gear becomes obsolete in 6 months just like vertical, but within that 6 months you have horizontal progression to wiggle around in