Whilst I agree with you on RTWs being a small fraction of overall revenue, codeshares are not about keeping people on an alliance. Codeshares are set up by carriers not serving a destination when they feel they can make additional revenue on a route they don't serve using their own code.

For example, if AA think they can sell a fair number of pax on a BA flight to CAI, they would set up a code on the BA flight negotiating how the revenue sharing arrangement would work. My understanding is that there are several revenue sharing models that can be put in place for flights coded by a codeshare partner. BA for example might suggest that the AA code entitles AA to 30% of the overall cabin and therefore AA is responsible for selling tickets up to that 30% mark although providing a significant margin (but AA also have a greater share in the risk in this model). Another alternative would be for BA to retain control and simply provide a small margin to AA for flights placed on the AA code but for BA to retain overall control of sales. There have been some excellent and more detailed posts on this topic before on both this forum as well as the QF forum if you want further information, although in essence, codeshares have little necessarily to do with alliances.

FWIW, a JSA is a much more integrated approach but discussions of JSAs are better left to other threads.