In many languages the question mark can be the only difference on print between interrogative and declarative sentence, and as my native language is one of them, I hope that English will develop in that direction.
English can work that way too, converting statements to questions with inflection or the addition of a question mark. e.g. "You understand?"
However, in this case, interpreting this as question rather than a statement leads us to see a question about the inner thoughts of the commenter, which clearly is not what was intended.
What I believe was intended was to make a statement that was to be understood as uncertain. However, the "I think" prefix is all that is necessary for that and the addition of a question mark just creates uncertainty about the it being interrogative/declarative.
