One thing to be aware of is that the range of incomes in many countries is much more compact than the US. If you define social mobility by the ability to move from the bottom segment to the top, it requires a much smaller change in income in most industrialized countries than in the US.
For example, for the average person in Denmark to go from the bottom 10% to the top 10% of income requires earning a mere $40k more income. In the US, you would have to earn $90k more income. Consequently, it takes considerably more effort in the US to increase one's income in the US to the point where it would count as "social mobility" than in many countries even though you would be much better off in the lesser quintiles in absolute terms.
In short, there are many countries where moving from $20k to $40k is considered "socially mobile" but also others where moving from $20k to $60k income is not because the top bracket is $80k. Most people would probably prefer to live in the latter country. It may be a little harder to reach the $80k than $40k to meet the definition of "socially mobile", but in absolute terms they are far better off with $60k of income regardless of the definition of social mobility.
For example, for the average person in Denmark to go from the bottom 10% to the top 10% of income requires earning a mere $40k more income. In the US, you would have to earn $90k more income. Consequently, it takes considerably more effort in the US to increase one's income in the US to the point where it would count as "social mobility" than in many countries even though you would be much better off in the lesser quintiles in absolute terms.
In short, there are many countries where moving from $20k to $40k is considered "socially mobile" but also others where moving from $20k to $60k income is not because the top bracket is $80k. Most people would probably prefer to live in the latter country. It may be a little harder to reach the $80k than $40k to meet the definition of "socially mobile", but in absolute terms they are far better off with $60k of income regardless of the definition of social mobility.