I'd imagine that ship sales drop off because as account unlocks, one purchase is all you need. Then since the main influx of players tapers off, fewer need to buy them.
It'd be best to just convert to Zen and avoid all this confusion anyway. Not that the word Zen seems appropriate for Trek, but neither does Cryptic Points.
I'm not sure they would switch CP to Zen -- PWE seems to like having each game have its own particular in-world currency and Zen be the account-wide currency. (Easier to tweak rates for a single game with respect to what it's worth in RealMoney, I suspect).
STO has one of the messier economies I've seen in a game. EC, Dil, CP, GPL, Lobi . . .
"I weary of the chase. Wait for me. I shall be merciful and quick."