It seems a little like special pleading. The ease of transition from bottom to top quintile is an accepted index of how mobile or stratified a society is, not how wealthy individuals are. Of course different nations will be better or worse off, or will have different Gini indices as a result of social policy, but that doesn't mean we can't meaningfully compare their mobilities. Pointing out the figures isn't "propaganda".
> a 40%er in the USA is better off than a 40%er in Europe
Generalizing about "Europe" doesn't make much sense. Between (say) the UK and Albania, there's a vast economic gap (about 10 fold lower GDP/capita). If you're talking about Western Europe, what do you mean by "is better off"?
The relevant reduction in mobility would primarily be within the top 10% tail of the Pareto distribution for income, the dynamics of the bottom 90% are quite different -- although their absolute share vs. the top 10% has certainly declined.