The statement being "but if I had to choose a country to get ahead in life with the most opportunities possible as an immigrant, US is still the country to be at".

I don't think your rebuttal addresses the data I linked at all or supports the parent point at all

Upward mobility is definitely down and the USA is no longer the world leader by some measurements. I get that won't persuade everyone but I'm happy to debate on that data instead of an unrelated discussion point even if it is interesting.

If you measure mobility by the ability to rise from the lowest quintile to the highest, yes other countries do better. But the top quintile in the US is just leaps and bounds past anywhere else. Rising from the lowest to the 4th quintile is quite common, about 1 in 10. If you look at the 2nd quintile about 35% of their children end up in the top 2 quintiles. Reaching the 4th quintile means a median income of more than $120k, which by global standards is pretty amazing.

The median income of the lowest quintile has done slightly better than inflation since 1967, but the 5th quintile has as we all know gone up precipitously. The 4th quintile has beaten inflation by something like 30k. So rising from the 1st to the 4th quintile is still quite a lot of mobility.

Upward mobility is doing fine for people with parents at home, but this accounts for fewer people these days.