As a metric, "social mobility" does not mean what most people intuit it means. Someone who, in real PPP terms from the same base, doubles their income may be wildly "socially mobile" in one country and not in the other, even though their absolute outcome is identical. This is because it is a function of income distribution which varies widely and is partly a function of country size.
Many people who are "socially mobile" in a country with compressed income ranges, like Denmark, will be objectively worse off economically (PPP) than many people who are not "socially mobile" in a country with wide income ranges, like the US. Income distributions need to be similar for social mobility comparisons.
Most people readily prefer a "non-socially mobile" increase in income if it is greater in absolute PPP terms than smaller "socially mobile" income increases. In these comparison cases, which are not uncommon, there is literally no benefit to being "socially mobile" in a country with compressed income ranges.
Many people who are "socially mobile" in a country with compressed income ranges, like Denmark, will be objectively worse off economically (PPP) than many people who are not "socially mobile" in a country with wide income ranges, like the US. Income distributions need to be similar for social mobility comparisons.
Most people readily prefer a "non-socially mobile" increase in income if it is greater in absolute PPP terms than smaller "socially mobile" income increases. In these comparison cases, which are not uncommon, there is literally no benefit to being "socially mobile" in a country with compressed income ranges.