Odds of Hanging On
It depends. It's not easy getting in in the first place, and with newspapers in decline, it'll only get harder. That being said, the career isn't going to die tomorrow (well, technically we guess it could...it depends on when you read this) and getting in is typically much harder than staying in.
Historically, newspapers are pretty reluctant to give space to untried comics, but established strips can pretty much keep their space until the cartoonist quits or dies, regardless of how good (or not) it is.
Cathy Guisewite drew Cathy for over thirty laugh-free years, yet the strip was carried in around 1,400 papers at its peak. We're not aware of Hagar the Horrible ever causing a single human with greater than a second grade education to laugh, but it's still going strong after something like 162 years.