So long as TES games continue to make money, they will continue to make them... end of story.

Yes, they very likely are going to make an MMO at some point, probably before another single player TES game would come out. However since ZeniMax Online would be doing any possible MMO I highly doubt they would stop making single player TES games like BioWare did with KotoR... they likely will stick with the Final Fantasy strategy of "the more the merrier".

Skyrim is far from being the last TES game. It will be the last for this generation, but there will be many more, especially since we are now in the 4th Era. I believe Bethesda just wanted to link the events of the first four games together to show some level of cohesion and completeness. We also have to consider the MMO ZeniMax Online has been developing for the past couple of years which is more than likely TES. This is only the beginning of a continuing juggernaut in the video game industry.

Why would it be the last game? We still have the Black Marsh, Hammerfell, Valenwood, and Elsweyr to visit.