I stand corrected and certainly defer to your expertise on icebreakers.
AIVIQ was so thoroughly discredited by its becoming disabled due to basic design and construction deficiencies during the KULLUK tow, and it's role in putting KULLUK on the rocks --- which became the total constructive loss of the only proven ice capable oil rig available --- that it became difficult to say anything good about AIVIQ.
To be fair to AIVIQ --- KULLUK on the rocks --- occurred most because AIVIQ was incompetently crewed, handled, and managed by Chouest, Shell, and the USCG Incident Command team.
This not only destroyed KULLUK, it also ushered in a U.S. Arctic drilling regulatory regime that was designed to strangle Arctic drilling for the foreseeable future.
It appears that Shell permanently lost confidence in AIVIQ after the loss of KULLUK, but could not admit it. Instead of dispatching AIVIQ to other tasks somewhere in the world where it could prove it's worth, they kept her tied to the dock for two years where she could not get into any trouble. When Shell returned to the Arctic in 2015, they did not trust AIVIQ to tow the rigs.
As Drillbill pointed out, it appears that Shell was too afraid of another AIVIQ incident to really try her out, and to let her prove herself.
Now she sits at the dock in Seattle again with nothing to do, except to be disposed of.
If Shell had been smart, which it clearly wasn't, they would have found a way to prove AIVIQ's capabilities somewhere else in the world under competent management, with a new name and paint job, and they would have washed off most of the taint of the KULUK on the rocks incident before the 2015 drilling season.