>The "the young people have to work more" argument is only valid as they are working for the older generation.

Nope, that's just a tiny part of the problem.

An economy has a certain size, which depends on how many people support it (work) and how many people buy stuff (consume).

Fewer young people means (everything else being equal) less productivity. That's regardless if the old people are kept around or euthanized (!) or whatever.

Seems you forgot that declining fertility also means less young people each year, not just a larger percentage of older people. Even if you ...kill anybody above 40 years old, the number of 20 and 30 year olds will still drop because of the declining fertility.

Smaller worker and consumer base then means contracting economy.

I see what the issue is - you see a contracting economy as being a problem in itself. It is not, as another commenter pointed out.

Ah, ok, if another commenter pointed it out it's not a problem, I guess it's fine then!

Let's check back in 20 years.

Well, there is not reason to rewrite what another commenter wrote.

But I agree, let's check back in 20 years