> Sure, during that decline there's going to be issues, but once you hit a new stable population those would go away.

I think you're compressing a lot of hardship into "issues"... It's not about total number of people, it's about demographic distribution. If half of your population is at retirement age, who is going to take care of them? Who will be working to generate tax revenue to provide the social services that you expect?

Also populations don't just stabilize, they feed back off of themselves. If half of your population isn't procreating, you get fewer and fewer babies which is a vicious cycle. There aren't many policy levers to pull during demographic free fall.

Easy, raise the retirement age.

The problems you are expecting from demographic free fall aren't going to happen for generations, and at any point during that time people could decide to start having more babies.