I wanted to chime in on this.
Let's say TFP has a "development rate" or a certain amount of things they can accomplish within a specified timeframe, let's say one year.
At the moment, a vast amount of features and other tweaks are added per update. If they were to split the workload into two parts, as in two updates, but with the same amount of content, it would roughly mean that there would be two releases within that given time.
I think one of the big differences with this update as to updates they've done in the past, is they're literally trying to finalize their entire game framework. With so many "gut" components reworked or freshly built, A17 is going to be a bit of a unique update. In past updates, they've done similar things if I recall. But it was more... tinkering than really defining and designing. If memory serves, the guy who implemented the new/updated physics to get cars attached to the road properly, allow jumping vehicles properly, etc, was a new hire for A17 or just before A17.
Someone else was too... I want to say fataal but I may be on crack.
I think what A17 is going to give us more than cool new things to play with, is a real near finished or finished gut of the game. That's no small feat and nothing to scoff at.
When you do take a large application, with a lot of dependencies, rip out it's guts, and put in nifty new ones, it takes a whole lot of time. With everything I've seen them put into A17 it's kinda like this...
Imagine you have an old Ford Mustang. She's a beaut to look at, but the guts just are kinda not all together. The frame itself is rusting through. You can patch the frame or you can fabricate a new frame, with enhancements, and take off all the good stuff of that old Mustang and slap it onto the new frame.
Pre A17, seemed like a lot of frame patching. May only have to jack up the car to get this done.
A17, new frame fabricated with a ton of cool new features that lets you modify that old Mustang in ways you never could before. You can't exactly throw tires on the new frame and go off for a cruise. Kinda need to do the whole bit.
Basically... If there was content that was nothing but "Add Cardboard Box and Zombie Squirrel", sure, TFP could absolutely choose to push that out all by itself with minor versioning overhead headaches. Minor being a relative word here...
But that's not at all what A17 is. At least... that's my take.