There's also a political factor for why it's not being boosted, in that as long as the ISS exists governments and the public would want to see it maintained and used. Just boosting it and letting it rot would make space agencies look really bad in the eyes of the public. It's better to just decommission it and have no chance of a future salvage effort. (this doesn't touch on why not do privatization of the ISS, other comments have responded)