The cargo boxes do leak and require maintenance. This is especially true where holes are cut in them. If you cut as many holes in the cargo boxes as TC's and put in a slide or two, do you really think they would so robust?
The TC you are searching for exist, and has for several years. They are typically at the top of the line in TCs as it pertains to overall quality and price. They do not have the side cut open for a slide. I am referring to a molded fiberglass TC. If a molded fiberglass TC had no holes other than those in a Cargo Box, they woild not have any more leaks than a Cargo Box.
Put a Cargo Box in the ocean, I bet it sinks. Put a fiberglass boat in the ocean, it will not leak other than the hole in the top.
Wayne