Modern society is different, for better or worse, and it makes no sense to just throw open the gates and have anyone come in.

Do you have evidence for this claim?

“What can be asserted without evidence can also be dismissed without evidence.” – Hitchens’ Raxor

I don’t mind reserving certain things for citizens.

The vast majority of benefits or restrictions we have The State provide or enforce should be for all individuals.

In any case, citizenship can be queried at the point of receipt of those rare things, if for some reason we do need to restrict those things to citizens.