The threat from within Belgium is not unsubstantiated. It is the base from which attacks in France were planned and the terrorists were Belgian nationals. However I do not think we should react by imposing a travel ban on all Belgian nationals. This is what I mean by bad policy because the supposed security benefits (which are unquantifiable) cannot be shown to outweigh the damage that would be done to our relations with the innocent people of Belgium. The travel ban takes no account of these unintended consequences. It would be far better to acknowledge that we have a security problem and tighten border control for everyone so that innocent people who have business or personal reasons to travel are not prevented from doing so. I am advocating inconvenience over exclusion which seems like a more proportionate response.
Interestingly on the one hand you substantiate that Belgium might be considered a haven for some Islamists activity and therefore might hold a real threat to other European countries but you then go on to say that you wouldnt want to impose a travel ban on all Belgiums, but nor does Trump, we have absolute confidence that the EU's and Britains intelligence and surveillance services can protect its own citizens to a point that any threat might be contained and controlled, it is therefore wholly reasonable why we would not wish to impose any travel ban on Belgium citizens, so on this point we agree.
But you cannot then use this as some reasonable comparison of Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen.
It is a ninety day temporary ban, for a newly elected president that wishes to scrutinise the vetting procedure of countries that Obama identified as posing the greatest threat to the US, seems wholly reasonabe to me.