"I'm simply challenging your assertions that there will be no problem with an open border, as those assertions are based on assumptions that are in no way guaranteed to hold true in future."
Nothing is guaranteed to hold true in the future. If nations (or indeed individuals) declined to do things simply there was no guarantee that things would always be the same then nothing would ever be done. We cannot be sure that the EU will even exist in the future, let alone what changes it will make to its members' legislation. At present there is no indication that Ireland will change its border arrangements. The biggest threat is if the EU insists it becomes a Schengen member (which is more likely than not). If it does the UK will have to reassess its position (with all the concomitant implications that has for the Good Friday agreement). But it is ludicrous to suggest that the UK cannot make a decision because "things might change." That is why so little gets done in the UK. Everybody is terrified of making a decision "in case things change". Far better to kick the can down the road for a few months or years and let somebody else deal with it. Things are always changing.