The idea that applications should be consistent across OSes is not really pointless, it just happens that for some reason the webbrowser is now one of the major platforms for applications instead of java running in the browser. Also in the 1990s there was not a clear indication that x86 would dominate the whole market and in particular for non customer facing applications there really was no compelling reason why they had to look good or "native", so Java certainly looked like a good idea, as did Smalltalk.

The incentive for Sun probably was in part that they wanted their high end workstations to stay relevant and maybe eventually develop cpu's that accelerated java execution.