Thus won't happen and it is impossible for it to happen.
So immigration is just kicking the can down the road (with heavy societal cost) instead of facing the challenge head-on. That challenge is to accept that population will stop growing and even decrease and to adapt.
For instance, technology allows and will allow to automate many jobs.
Personally, I think we have quite a long way to go before we reach a point where that cannot ever be true and I think we should account for the possibility that humanity finds a way to keep expanding forever. Whether that be into the ocean or outer space or perhaps ever vertically on Earth. But those are problems for a thousand years from now and I don't think are as worthy of consideration in 2024.
What sort of other challenges do you see that need to be addressed first and foremost?
As said, birth rates are already below replacement rates in developed countries and decreasing everywhere else. Projections are that the global population may stop growing by the end of the century.
The seemingly "easy" option of immigration is causing big societal issues in Europe with a growing backlash against it.
The global environmental crisis is caused by the booming population of the last century or so combined with economic growth.
So this is very much a "now" issue.
Japan will be the first place where we will see this playing out. I think this will be one of the biggest social experiments of our lifetimes.
I do not know how it can be done, but I do not see another way without huge productivity gains via technology to have some sort of support for an aging/dying population.
They are unofficially bringing in plenty of Vietnamese, Indonesian, Filipino, Nepali, Thai, and Pakistani immigrants via ill-disguised migrant worker programs that have Gulf level abusive conditions
Same with South Korea and Taiwan.