What I am questioning it your sides apparent claim a sunset clause has to be in the initial bill to enable the bill to be modified in the future. I mean it is OK to defend the sunset, I just don't think "so it can amended later" is a good defense.

Here's the procedural justification for sunset clauses like that:

The easiest result to achieve in any legislative body is to do nothing, and that's especially the case in the U.S. Congress. We have a bicameral legislation, with a committee system and a Speaker/Majority leader that is entirely empowered to keep bills off the floor, and a presidential veto, which means lots of opportunities for people to stop a bill from passing. It's hard to get substantive legislation passed, even if there's a lot of substantive agreement on it.

Sunset provisions force action. They force a bill to be passed just to maintain the status quo. So if you put a sunset provision on a measure that you're pretty confident people will try to extend in the future, you create a lot of incentive for a modification bill to not get bogged down in any of the veto points described above. It gives everyone incentive to pass another border bill in three years.

Again, I think the main reason for this sunset clause is still that blocking refugees from even asking for asylum is a clear violation of international human rights law, and so the sunset at least gives the U.S. a fig leaf to argue that this is a temporary violation just in response to a short-term crisis (and not a U.S. abrogation of our human rights obligations). But that's one of the reasons why sunset clauses are deployed generally.