I'll try to explain my point of view on why the US and the EU wouldn't save Ukraine from default.
1. The US does not really care about Ukraine because its goal was to quarrel Russia and the EU and create stronger economical ties with the EU. And the goal has been achieved, the EU wants to reduce its dependency on Russian energy and would probably conclude the Trans-Atlantic Union with the US. Accepting Ukraine to NATO is not possible at the moment, so preventing default won't help in that, no point to spend money on that either.
2. The EU wanted Ukraine as a large market for European goods and this is definitely not happening in nearest future
3. The EU also can't afford to spend money on Ukraine because it has economical problems and it needs to bail Greece once again
Russia holds Ukraine pretty hard: gas bills, debt needs payments, cheap coal and electricity imports. I don't think the EU or the US would pay all those debts and more to save Ukraine.