Yeah in theory you would be correct. However in actual history this isn't how they work. As a matter of fact DWTI had no decay from June to January last year and actually had a boost in performance beyond what even a pure 3X play would have paid.
So had someone bought last summer DWTI but sold because of "decay" they would have missed out on a huge winner.
Also a strange but true fact is buying equal amounts of both DWTI and UWTI in June and holding till January would have made you +100%. With decay you should have lost money right? It didn't happen.
If either of these moves sideways for more than a few weeks simply step off the bus and wait for the next round.