Thanks for the update @Piranha.
Interesting that data security and cost are the sticking points since, in my opinion, those are not Tableau's strong suits. For what it's worth, I've set up both Shiny servers and Tableau in parallel at the same company. Tableau more for self-service analytics case and where we spent the time to curate the data and expose the right elements ... and where the analysis was simple and straight-forward. Shiny was used when data still required a lot of manipulation not easily or impossible to be done on Tableau and where there's modeling and analysis; things that are not appropriate for self-service.
The 3rd factor, IT not having experience with a software so not wanting to do it, is a common one. I've always been willing to get my hands dirty and be responsible for administrating and working with IT, so I haven't had as much push back in that regard.
Good luck and keep us posted.