... Design might be subjective, but its effects can be observed objectively through data analysis...
I like your overall post, but I wanted to highlight this part in particular. Data analysis by itself is only useful for quantifiable items. Things like repeat play time, and the specific habits therein. Past that, and you're extrapolating; "players play more, therefore they enjoy the game", for example. It sounds obvious, and I'm sure you're aware of it, however it is worth noting and repeating that this only tells a portion of the overall picture when dealing with something like a game, and a reliance on strictly data driven game design can have overall negative impacts.
To put that into context, the original Destiny had an overall design goal of high repeat play time. A "hobby game", as Bungie put it. To help meet that goal, Bungie obfuscated the identifying process for the legendary, or purple, engrams, to extend the gameplay loop/loot grind. Purple engrams had a low chance to actually produce purple items. From a data point of view, this met the intended goal: players played more, and kept playing, to acquire the loot. Mission accomplished, right? Unfortunately, it introduced a level of abstraction to the process that players found un-enjoyable, despite the amount of time they played. That disconnect is difficult to quantify. Bungie ultimately re-worked the process to improve player satisfaction, even though it shortened the loot grind, and triggered more players to exit the loop. Subjective feedback is a valuable tool for making the game simply enjoyable.
It's no coincidence that Destiny's "addiction" mechanics drew a lot of discussion when the game first launched. Bungie clearly applied a strictly data driven approach to their design, and have had to re-adjust the game and it's mechanics to account for subjective, difficult to quantify, feedback. It's not uncommon for people to say "I put hundreds of hours into Destiny, but it's a poor game". A strictly data point of view was used to modify, or balance, the PvP element as well. I think its fair to say that this has had divisive results.
