Chronic and Haji cover it fairly well, but in case you haven't seen or heard Why sbs is using low poly graphics instead of developing every thing in full fidelity graphics at this time, I did a faux pas and stole this quote from caspian on discord.
The TLDR of it, they found a happy middle ground that allows them to do thorough testing with a quick turnaround time implementing new things. The pre-alpha client is no way indicative of what the game will look like at launch.
Caspian ☁Last Friday at 2:49 PM
Games are a combination of art assets, engineering, and design. Each of which require varying degrees of scaffolding and iteration in order to create a final product. To develop games quickly, it's important that no team is ever blocked or waiting on the work being done by another team. Time is money and the more we can parallelize the work, the less money it costs overall.
To that end, many game studios engage in various forms of prototyping and gray boxing. The degree to which you prototype or "gray box" depends on what questions you're trying to answer and what the minimum amount of effort is required to find that answer. Sometimes you're just trying to find the flow of a map or quest and sometimes you're trying to identify the different attack techniques in a new martial style. The more granular, precise, or specific the question you're trying to answer, the more detailed the asset needs to be to answer the question. And the devil is in the details, so they say. Much of what makes CoE the game it is, is our attention to detail.
Sometimes you can get away with using a broad array of literal gray boxes, such as when blocking out a dungeon or other environment, and sometimes you need something more specific, such as when testing collision mechanics that borrow much of their enjoyment from the skill required to hit a collision volume. Sometimes what you're testing isn't art related at all but are just trying to create models in which the animators can rig against in order to figure out the direction they want to take an animation system.
Of course, we want to make sure that any time we create low-poly assets that we're getting as much value from them as possible. So in addition to being used for rigging against to create more accurate animations than could be done with gray boxes, we also use them in the creation of multiple LODs, and we occasionally use them as base/starting assets for other derivative assets. Much the same way potters create molds of clay that can be used as the starting point for similar pieces of pottery work.
At the end of the day, using low-fi assets rather than pure gray boxes allows us to test and validate many scenarios we couldn't effectively test and validate otherwise, and also acts as the base assets for our higher polygon assets and LODs. By creating low-poly, low-fi assets quickly, and handing them off to the designers and engineers, it allows them to make forward progress unhindered, which would normally be slowed down by the time it takes to make hi-poly assets, materials, and textures. So the low-poly assets allow better parallelization, while still getting closer to the target output, and thus creating more accurate results.
It's really a simple formula: the closer the asset is to the final asset the more accurate the work, but the longer it takes to make the asset, the more throw-away work, and the less parallelization that can be done. In contrast, the simpler the asset, the faster it takes to make it, the more parallelization that can occur, but the more likely your results are to be incorrect.