Let's just give the terrorism angle a rest - and the deep suspicions centred around, "they aren't telling us everything".
1. Terrorists make claims shortly after terrorism events to maximise the terror impact. No such claims have been made, apart from one online claim, from a previously-unknown group within China, that the Chinese have deemed a hoax.
More importantly, world intelligence agencies have picked up no increase in known terrorist groups "chatter" - a common happening immediately after a successful terrorist attack.
2. Of course, we "aren't being told everything"! All countries possess secretive defence agencies and installations, and they're not about to let it become public knowledge what they have, and what they are capable of.
Probably more importantly, they don't want it to become public knowledge, what they failed to pick up!
It is highly likely, that even if MH370 flew within range of a military radar, spying setup, or other military establishment, it wasn't picked up - due to lack of operator alertness, even perhaps even, unexpected downtime of the facility.
The simple facts remain that the aircraft most likely suffered a major electrical fault or power outage, or a rapid decompression caused by a cockpit fire or other reasons, leading to rapid crew incapacitation - and the aircraft flew on for an extended distance, well outside any radar coverage, before it finally splashed down in the vastness of the ocean.
Only those who have actually carried out SAR searches over the vastness of the oceans, fully understand how much even a large airliner becomes a "needle in a haystack" out there.