My shilling's worth:
If the round trip fare ends up being US$400 to BZE, plus the US$125 or so RT fare to/from San Pedro, plus the extra Belize exit fee, I doubt Tropic would do much better than Maya Island did, and Maya was flying larger equipment that appeals to more international travelers than a Caravan.
US$400-$600 is about what it costs to get to Cancún and back from a lot of U.S. cities. Doubling the fare to get to San Pedro or to Belize City is not going to be appealing for the average tourist. There's a market for the convenience over the US$80 RT ADO express or ADO to Chetumal then water taxi to Caulker or San Pedro, but it's pretty small. For that money, it would be cheaper and easier to fly direct into BZE from the U.S.
It's not a value proposition to fly a tiny plane on a two-leg journey to San Pedro at a cost about the same as coming to Cancún from the U.S. or Canada.
Despite the obstacles, I think the San Pedro entry point would lure more visitors.