PowerBI may be subsidized or not. But in our use-cases and in their documentation, it really doesn't funnel customers to SQL Server. Sure, it plays nice with SQL, SSAS/MD and SSAS/Tabular. But it also plays nice with many, many other data sources individually and in mashup. Its internal data modeling and ETL capabilities rival the power of SSAS and SSIS and they are free -- I see them as more of a competitor to SQL than a front end to it.
You may, but that's not how the strategy guys at Microsoft see it, according to their own words. Data modeling and ETL capabilities rival SSAS and SSIS only in very trivial scenarios.
I'm not saying that to bash on PowerBI. I love that product and use it heavily and promote it to clients. It just serves different purposes than Tableau.
The data modeling engine IS SSAS. The only major missing pieces are row-level security and defined KPIs. The query language, DAX, and the backing columnstore database are the same engine used in SSAS Tabular.
xVelocity is only a (small) part of SSAS and it has a very limited application compared to OLAP even in the upcoming 2016 edition which greatly enhances the columnar store functionality.
