"What are the downsides to abandoning the ISS aside from a gap in the science data?"
The end of the ISS.
The ISS has a low orbit that skims the atmosphere reducing its speed and thus the height of its orbit, without constant station keeping it will literally fall out of the sky. It also requires constant maintenance to keep it functioning. Abandoning it for several months would mean abandoning it permanently.
Fourteen months is doable. Valeri Polyakov spent 437 days and 18 hours on board Mir in the mid 90s.