First, it's hard to imagine such a world. Under normal circumstances as productivity/automation increases worker labor will always be an increasingly smaller part of the value added, which means capex grows relative to opex which drives r.

The way r can get suppressed long-term is through (1) wealth taxation, (2) depreciation of an important asset class (eg as birth rate falls further below replacement housing can start to depreciate), or (3) high inflation.

If it is (1) then there really isn't much to be done, although hopefully the redistribution will be administered sufficiently well to help everyone achieve a high quality of life. If it is (2) then you can avoid the depreciating assets. If it is (3) then you once again avoid depreciating assets (don't hold cash/bonds/cash-like assets).