Here we agree: the verb tense is crucial to what Jesus was doing in the context of that story in Matt 22:31-33. And, his hearers fully understood what he was doing by choosing that tense, which is why “they were astonished.” The Sadducees denied the bodily resurrection in the last days, and Jesus was showing them that they should know better by stressing the present tense in his own reading of that verse in Exodus, by adding to the original the further truth (not directly stated in Exodus) that “God is not the God of the dead, but of the living.”