You are assuming the ideal condition in movement of UVXY in either direction to expiration. Simply put, what if UVXY move to either $10 or $21 in just a couple of days, you do the math. From now to Sept expiration is a very long time to predict the movement of any stock.
If it goes to 21 in two days or expiration it is all the same from the options standpoint. Actually, it might be even better, especially if you get exercised. You get your max gain in the shortest time possible. Only the slide to 10 in two days would be a bad thing. Then you need to exit the trade when options still have the most extrinsic value. However, since it is moving away from the strike, it will still be lower than the purchase price. So only the quick down move is a bummer, while the quick up move is a supreme win. You can enter another trade in the same time frame. As a rule, however, when selling (writing) options, you generally want a shorter time frame so it gets to expiration more quickly. When you buy, that's when you want the extended time frame so you give it a chance for it to move the way you want. If you are interested in options, VXX has weekly options that are fairly easy to master, IMO.