Regarding the population part, I don’t think the population sim in this game is detailed enough to support such feature. Not even sure the game is intended to simulate such long-term effects.
Arguments for ‘unreachable max GDP’ have been a constant topic for discussion. I think the right question is ‘how’ rather than ‘why’ as of now. Cliff or any modder should be able to reduce all the existing GDP boosting effects to just one fifth but that’s not going to fix the problem IMO. Democracy is about comparing possible choices. If one of them is seemingly too trivial, it could be easily neglected.
I’d like to see GDP boosting effects to be more specific. Introducing several industrial sectors and some macro-economic notions will make it harder to stimulate the economy with a handful of policies and perhaps allow some conflicting effects on the economy, making some economic policies situational. But this isn’t as simple as ‘just one-fifth’ idea so I don’t expect this to arrive in near future.