They ruled out the Northern corridor only because they said there where too many radars in that region and speculation that “somebody” would see the plane. Not necessarily.
Assumptions for the Northern route are absolutely false. Just because a country has radar capability doesn’t mean they have it on. OR, it they are complicit, (i.e. Pakistan) that they would even report it. All they have said to date (publicly at least) is they saw nothing. That doesn’t mean they acknowledged not looking.
Just to pass the time...
Go to flightaware and plug in Singapore flt. 68, then overlay that track with the track of fl370 after the turn to the NE.