With all due respect, there are countless things God has done and is doing and has yet to do which I could not have imagined. My thoughts are not His thoughts. My ways are not His ways.
Frankly, I could not have imagined the Almighty subjecting Himself to the whims of his creation in order to save those who rebelled against Him. Still, He did just that. The message of the Cross is foolishness to those who are perishing.
How did David make it to Heaven?
And if I may add some clarification, "fulfill" here does not mean "bring to an end by fulfilling" as some believe. It can have two meanings. The first is simply to "make full" or "make complete." All the Torah points to the Messiah in one way or another, and by His presence, we now have the keys to understand it.
The second, rabbinically, is, "To properly interpret." This interpretation would seem to follow from the fact that the next four chapters (Mt. 5-8) are a sermon on the proper application of the Torah rather than a dismissal of it or an explanation of how Jesus intended to "fulfill" it.
Not that keeping the full Torah and converting to the Jewish culture is a condition of salvation or fellowship (Acts 15). But I think the Church as a whole for most of its existence has been wrong in simply dismissing it, or making disobedience to certain parts of it (like kosher) a condition for Jews who wished to enter into the fellowship of their Messiah. ("Oh, you're a Christian now? Wonderful! Here, have a ham sandwich!")
>> So, David, Moses, and Elijah aren't in heaven? <<
No, they were not until the resurrection of Jesus. They were in Abraham's bosom, a mild plane of Hades/Sheol/The Grave.