It's an explanation, but it's not correct. Any given feature is equally likely to become more or less complex over time. That's the entire premise of variability. The reason that we see more complexity with time is that there is a lower limit on how complex an organism can be, but there is no upper limit. So, statistically speaking, over time the average tends to be more complex. However, complexity has a high cost (in terms of energy usage), so where lower complexity is possible without sacrificing fecundity (as in the cave example I gave above), it will likely be favored. Don't fall into the trap of teleology, although it's easy to do (as we all think the world "wants" us to be here). The arrow of time moves forward, but the arrow of evolution doesn't exist. Full stop.