I see a number of problems with interchangeable TTL modules. The first one would be allowing an interchangeable module while still maintaining a sealed unit and keeping the size of the strobe down. I think those to things are mutually exclusive. The second drawback is you'd need to buy the interchangeable module for each strobe you use. I use 2 x DS-125 so I'd need to buy two modules. Third, you'd may have to have dedicated sync cables depending on which interchangeable module you are using.
With the current system where it is built into the housing, everything from bulkhead on the housing is exactly the same regardless of what type of camera you have. The same sync cables are used for Nikon, Canon or whatever. The strobes are the same. Everything is simple and self contained. The logic for converting from the camera circuitry is in the housing which is made specifically for that camera which is the most logical place for it. If the D700 was to introduce some new iTTL feature, that could be built into the D700 housing and nothing else would need to change.
Now, if the external TTL adapter is not reliable (in your opinion), why would an interchangeable module be any more reliable?
How often have you told Ikelite what you want in a strobe? How many other people have told them the same thing?