I think you are ignoring the core issue. The problem is not so much the population decreasing but it getting older. If the average age goes up (and therefore average productivity goes down significantly) that will likely outweigh all of the potential advantages you have discussed.

What is more, unless we change how democracy works (e.g. young people and those who have children (<18) get extra votes the older generations will keep voting for policies that only benefit them and make things even worse longterm.