Any improvement, however small and however late, is to be welcomed for what it is. Extra usage will provide extra revenue - and extra pressure for further improvements. Is this isn't seen as a "success", than that reduces the likelihood for any further improvements.
I've some experience of the Prague public transport system , probably very similar to Vienna's. That will have been up and running at a time when car ownership was at a much lower level than it is now. It wouldn't be easy for anybody to justify the expenditure or the disruption that would be needed to build it now.