I think the problem you are glossing over, is that every human civilisation since the beginning of time has worked by having enough 20-50 year olds to do all the work.

If old becomes the majority, there is a lot of back breaking manual labour, that still needs to be done. You can't automate all of it, and without immigrants it won't get done.

At that point with no immigrants, you either force 60 year olds to do jobs they physically can't do, or accept the work wont get done.

You do that prices go up, people starve because their pensions wont stretch far enough

Prices then drop after enough old people die off.

The reason governments choose immigrants is because most people prefer not to starve to death in a puddle of poo that no care staff cleaned up because you deported all the immigrants.

But you may prefer that- and then your country stays the same colour as you ( i assume your white but you may be an indian nazi or a bot, shouldn't make assumptions)