Bear in mind that there is no "real data" of the future. There is only present and past data. And modeling based on that, well, let's say there are no guarantee

Bear in mind that there is no "real data" of the future. There is only present and past data. And modeling based on that, well, let's say there are no guarantees in life. Part of planning is understanding where the risks are, what the premises are that the model relies on and so on. Depending on inflation being 0-2% over a 30 to 40 year working-life seems optimistic. So, in planning, some diversification is appropriate. Some part of your retirement earnings should be inflation-adjusted. For example say you get rent on a property. That rent will tend to rise following inflation. Equities, especially ones chosen for dividends will also tend to follow inflation (over the long term). By contrast a fixed-yield item like say a bond returning 5% is not inflation linked, and leaves you vulnerable to inflation variances. If your model only relies on fixed-yield, and does not allow for variable amounts of inflation, then the model needs updating IMO.