If we don't limit immigration then of course we will have a population explosion, that's the point of limiting it.
Again, if we bring more people in which pushes the population up then what happens a few generations to come? What we do now matters, if we don't limit immigration then we build up an ever aging population to a point where we may never be able to manage it.
Decisions have to be made, either we don't allow old people to get old or we convince the population that's here to have more and more children who will be of age to look after the older
Then the population gets larger and the problem could be bigger and bigger. people.