I honestly don't think Sink is a bad candidate. Her 2010 result was pretty strong, and it's not like she got blown out in the House election either (a non presidential year special election would've been tough for any Dem). Now, if Florida had a stronger bench, I'd still not want her to run for anything again. But since it's Florida, she might actually be one of our better candidates...
You're sugar coating it. Both of Sink's loses were an embarrassment. She shouldn't run again, Democrats should just put a fresh face out there.
How was 2010 an embarrassment? That year, Republicans were winning by double digits in states like Michigan, Pennsylvania, and Nevada. Florida is redder than those three (especially in midterm years), and Sink lost by a point, and even held Scott under 50%.
Her 2014 loss was more of an embarrassment simply because she started with such a huge lead, but that was obviously never going to hold in an R+1 district in a low turnout election.