But, there are two points here:
1) Should a person proposing something have the burden of making the case for that proposal? Of course. You know I agree with you. This is the point you are pushing, and we agree.
2) Will Trump presenting an expert change any anti-trumper's mind? Of course not. Sure, it checks a box. But, they will just accuse him of being as biased as the expert that Obama trumped up (pun intended) for health care reform. This was my point. Surely we agree on that as well?
In this case, I think it is basic common sense that a wall would be a component in border security. You wouldn't require an expert to testify under oath that locking your front door is a component of home security, would you? Can't the fact finder just take "judicial notice" of that fact? It seems silly for you to ask for an expert about the effectiveness of something as simple as a wall. Do we need someone from MIT to testify under oath for that?
And, I will say it again: effectiveness is measured at the margins. This isn't a binary issue. There are always people on the edge about making the illegal crossing, and any impediment will eliminate those people who were on the edge. And, there are always people who will bring wall scaling equipment with them.
By the way, I know you are a lawyer. I think the testifying expert is the biggest abused feature of our court system. Each side can always get an expert to say what they want. Why not just let the attorneys argue the freakin' case, and let the jury decide? Why force the parties to engage in the farce that our "expert witness" system has become? We know it drives up cost, but I'm not sure it makes the outcome of the cases any more just. Justice becomes more expensive, but the likelihood of a just ruling is not necessarily increased...again, because you can get an expert to say anything.