... That's more or less history when they can move to the UK and earn pounds which are worth even more than USD.
I agree with your post overall except for the quote above, very flawed comparison. So it takes $1.5 to £1 +/- a penny or two. What's that got to do with anything??
That's like saying people are better off in Latvia than in the UK because it takes £1.20 to 1 Latvian Lat. "Lats are worth even more than the UK £" so it must be so.
However I'm pretty sure the average person in the UK has a higher living standard than a person in Latvia.
So yes, I agree with you that marriage for a green card is not as common anymore (still happens though) however your currency comparison is not very logical, apples to oranges...