AP Human Geography time!
First of all, the problem isn’t the fact that there wouldn’t be enough people. There is enough people.
The main problems that could occur are due to the changes in the dependancy ratio. The dependancy ratio is the ratio of nonworking to working individuals. As a larger overall percentage of the population is above retirement, each working person must support more people who don’t (often can’t) work. As for now, this isn’t a major problem. There are easily enough immigrants to make up for any decrease in native-born population.
Some other effects include more job openings for migrant workers (workers immigrating from other countries), or at least a better jobs market, as there is less competition when there are fewer competing people. The unemployment rate decreases.
Another side effect is that there might be more social tensions, as people are now under more financial stress to support more people.
The increase in migrant workers might also cause anti-immigration feelings, as immigrants are competitors in the jobs market, and there are also cultural differences.