You can argue all you want about underachieving, but if the minimum bar for achievement is Sweet 16 or we've underachieved, that's going to leave many spoiled, entitled Husky fans miserable most of the time. We finished in the top 32 in 2016, a significant improvement over the NIT finish in 2015.
Underachieving is relative to the potential or expectations of that team, not relative to previous teams. The 2016 team should have been better than they were during the regular season. They underachieved.
Over the last decade:
2009 - Achieved expectations (were expected to be dominant wire-to-wire and were)
2010- Underachieved badly
2011 - Overachieved greatly
2012- Underachieved badly
2013 - Overachieved
2014 - Overachieved after a regular season that was about to expectations or a little below
2015 - Underachieved somewhat
2016 - Underachieved
2017 - Underachieved
2018 - Underachieving