Generally a "rule of thumb" is that MS *may* (operative word, there, "may") shorten your life span by 6-7 years. (joke to follow): So if, before your diagnosis, you've figured out just how long you're going to live, back that up by 6-7 years, and you'll be on target.
In other words...maybe shorten your life, some. And as you identified, folks with more severe MS, with chronic-progressive or progressive MS, aren't as fortunate..their lives will indeed be shortened.
I think these days with the disease-modifying drugs, those estimates should change, as many of us "old timers" (I've had MS for 25 years myself), even starting late on the drugs (I started 15 years ago), are doing better than before those drugs were available.