The problem isn’t birthright citizenship. Birthright citizenship has been in place a long time. The problem isn’t even immigration levels - soon the US will be constrained not by quotas, but by desire of folks to immigrate here.
The problem is welfare. Prior to welfare, birthright citizenship wasn’t a problem. Now, it is because it qualifies you for free bennies.
The real ticking time bomb is countries that aren’t friends of the US sponsoring it’s loyal citizens to come here and have a kid making that kid an American citizen.
The parent and child then return to their home country where the child is raised as a loyal citizen to that country and trained as a soldier and an enemy to the US.
At 18yo that child then comes to the US as a citizen and brings that America hating training with them, and there is nothing we can do to stop them.
You can end up with thousands of fanatical foreign soldiers in this country ready to wage a civil war.