I love its meta sense of humor, but there's a fine line between parody done right and going full 'Scary Movie,' where a funny concept is beaten to death by endless, increasingly 'wacky' sequels. I'll avoid saying 'reboot,' because many still interpret that word as blasphemy in the gaming space, but I think at this point, Saints Row does need something of a fresh start. It's easy to see that if they really wanted to, Volition could have stretched the idea into Saints Row 5, but with a map half a the size of the last game's and now a non-customizable lead character (Gat), these steps backward don't really scream 'sequel.' While it's good that Gat Out of Hell is going to be a healthy-sized expansion for only $20, I think it's time for Saints Row to seriously consider starting over. Now with Gat Out of Hell, at least a DLC concept is remaining just that, DLC.
This was also about the time THQ was self-destructing, so the need for Volition to cut costs by reusing their biggest asset, the map, was probably done out of necessity, not laziness. The alien invasion idea started as DLC, but was eventually adopted as a full-on sequel for the game. There are likely pretty good reasons for this, however.