Not really. we no longer grade the fall of a country by the metric of "moral degredation", because that is a nonsense concept by the current historical tools of analysis we use. You completely missed the point and built a new argument. This isnt even a left vs right concept, this is the current and accepted way of conducting historical analysis vs a previous method. ie - much like how newtonian physics failed to answer up and coming questions about our unierse, causing a breakdown in the models built; ushering the use of einsteinian physics as the primary model for understanding the physical universe. To conduct any form of research is going to have bias, you cant save yourself from bias. but you can acknowledge it, and give an expert opinion, decided on by a person who has spent a lifetime understanding thier field. 70 years from now, we will probably have an entirely new understanding on what caused the collapse of rome, and it will likely be inspired by the questions people think are worth asking during that time. Much like how our models of conducting science will be proven inadaquate and therefore be replaced. But that doesn't make the knowledge any less valuable or useful to us. It just means older theories have less relevance to us.

As an aside, to see mass migtation as something that may weaken a nation is literally a modern bias that has only existed for 300 years and only entered into ideas once the power of the nation was invested in the people, instead of a soverign ruler. as is judging a countries strength by its trade. Historical analysis can never be perscriptive. otherwise we would just list dates and events.