I think Tableau competes more with Power BI than with SSRS so I'm not sure that comparing SSRS and Tableau would be realistic. 
I think SSRS (and Power BI for that matter) are going to be fine in the near future. I don't know if anyone can say anything about any product in the long run as things change so often and quickly in the IT world.
SSRS itself has been competing with alternative products for over 15 years and has done a pretty good job. When it first came out, I would guess it could have been Crystal Reports and Active Reports as the leaders. SSRS did well in gaining traction against those and other products.
They all have different strengths and weaknesses but Microsoft manages to keep their reporting products fairly easy to use and relatively cheap. But like I said, I really don't know if anyone can predict what things will look like in the long term. Who knows...could be something like Pentaho taking the lead.
A lot of the reporting skills are transferable from one product to another - there are a lot of similarites. You have SQL as a good foundation. That along with SSIS and SSAS, I probably wouldn't be too worried if I were you.

Sue