We are always living in some kind of “now”. Our view of the world is forever locked into a “present” or illusionary presence. And in this way we habitually prefer the “synchrony of X” over its diachrony. What can only happen in any interpretation of “the history of X” is forever a reading with now in mind when now never was there in the past. Retrospection is never free from this now, forever placing itself into the past when it was not there. That is why in retrospect everything seems obvious.
To be pure historians of Buddhism we must place ourselves in the past without the baggage called “knowledge” into that time and place. The further back in time we go the less accurate the representation becomes. For example, Chinese Buddhism of the 4th century knows nothing of its advance into Japan the next centuries. It is not, “it had not reached Japan yet”, but rather, “did it know it was going anywhere to begin with”. I doubt Buddhism in the 1st century BCE knew it was entering China either. Buddhism simply had no plans. These are projections onto history after rather than before or during the fact. The interpretation of history continually changes as a time pushes forward, added to by more and more retrospection. The more we think the more the past changes.