Winnonlin is one such tool - been around forever, written in fortran. One of the reasons it is still popular is for regulatory use it's very well understood.

Winnonlin is one such tool - been around forever, written in fortran. One of the reasons it is still popular is for regulatory use it's very well understood. I'm not enough of an expert to answer the second part of your question - on why new numerical stacks are beneficial. It may be around building more complex models of PK/PD but I'm not up to speed on what winnonlin can and can't do.