As someone that spent about 30 years overseas for Chevron, I thought I might share a few personal views on international assignments:
In general, I found that vast majority of expats were technically well qualified and, in general, could compete on even footing for positions in the US. Far from being washed-up or "has beens", they were dedicated individuals that, for a variety of reasons - money, sense of adventure, advancement, etc - chose to pursue an international career.
It is quite expensive - on the order of 5 to 10 times more than a domestic relocation - to move a family overseas. Include the additional cost for home leave, housing, expat premium, etc and it becomes obvious that one selection criteria is whether the employee AND his/her family can cope with an international assignment. To be fair, there have been failures which is why the selection committee considers adaptability a key criteria.
It is a fair criticism that some expats, especially those in 28/28 assignments, overstay their welcome. This was particularly an issue in Cabinda during the 80's and 90's. For the most part, CVX has learned from this experience and, to a large extent, this was not routinely repeated in TCO.
While the premiums and other monetary compensation may seem over the top, trust me - you earn every penny your paid.
Happy to discuss further if there are questions