I have many friends who are Chabad, meshichists and non and I have had great Chabad experiences.
Without getting into the question of if Moshiach could come from the deceased, I present the view of many in the non-Chabad world.
There is a famous vort from a Godol of the last Dor, he asked how can Moshiach come in this dor? If he has a knitted Kippa, the Charedi will not accept him, if he has a down-hat, the Chassidim won't accept him etc. He explained from Esther - ain Esther magedes moladta... that everyone thought she was from their nation (Gemara). So too, he explained, that when Moshiach will come, everyone will accept him, everyone will connect to him and respect him.
If you are Chabad, you can claim that all religous Jews held of the Rebbe as a Moshiach or you can say that those who don't are not Jews, they don't count. I will not answer you either way, I think the vort stands. Maybe the Rebbe was a great man, but non-Chabad members will say that the reality is, he was not accepted by most of the dor. That is one simple reason why they do not consider him to BE Moshiach. If he comes back then maybe everyone will think he is Moshiach but the same can be said for any Gadol or lehavdil elef havdalos, Bob. Until that happens ...