Obama called for a cease-fire in the war in Ukraine. He also called for the crash site to be left intact so that evidence is not tampered with.
The latter is already a dead letter. The site is already contaminated as far as its evidence value goes. The Russian proxies have controlled the crash site from the beginning and have reportedly already found the black boxes and sent them to Russia for investigation. The proxies have been combing through the wreckage, moving it and tampering with it. They are very likely combing the wreckage for any remains of the missile that brought the aircraft down. That evidence could be used to determine whether the missile was fired by the proxies or by the Russians themselves.
The former, a cease-fire in Ukraine, is probably the worst idea that President Obama could have proffered.