Yeah but remember that these sort of toys are opportunistically designed, not purpose-built (like say Generations Springer, Metroplex etc.). The nature of the characters with limited fan appeal means that it's not financially viable for Hasbro to fund the development of whole new moulds. Rather existing moulds are looked at and then ideas about how to retro-fit them as new toys are devised based on redecorating the base toy. Someone obviously looked at Terradive and noticed the facial similarity w/ Depth Charge and the fact that he's armed with a trident, a common fishing weapon; and the design/shape is an obvious homage to Poseidon/Neptune's trident. The pectoral fins of rays are often described as being wing-like (and indeed BW Depth Charge's intermediate vehicle form is an aircraft), so having him transform into a jet and having wing kibble is reminiscent of the fin-kibble on the BW toy.

As for Rampage; alright, perhaps a crablike tank would be more suitable, but does one exist that you think that could be retooled/repainted as Rampage? TF Prime Megatron's jet mode is sort of crab-like, and the robot body does have that big imposing bulkiness, particularly with the broad chest. As for the weapon - I don't see how that's an issue.