Being short of funds, I can understand a decision to sacrifice a little history by replacing a trestle with a culvert if the culvert option is cheaper. But it may not be cheaper.
Say that repairing the trestle costs $8000, and replacing the trestle with a culvert costs $7,000. There is no question as to the cheaper option if you leave the loss of historical fabric out of it.
Now say that repairing the trestle costs $8000, and replacing the trestle with a culvert costs $10,000. A manager might still choose to replace the trestle on the basis that the extra $2000 spent today will be offset by the avoidance of future trestle maintenance cost. And yet this future cost can be hard to predict, so the decision becomes much more subjective. And there is still the loss of historical fabric to consider as well.
In this scenario, if a manager were not concerned about losing the historical fabric at that level of detail, and generally, preferred to modernize the track infrastructure to eliminate the risk of such things as derailments, trestle fires, etc.; that manager might opt for replacing the trestle even if the up-front cost were higher than keeping it. At this point, the decision is influenced by individual mindset, and no two managers would necessarily come to the same conclusion.
That is why I asked about the cost of repairing the 2nd trestle versus replacing it with a culvert.