Conversely, I would not be shocked to see the vast majority of multi-linguals to be low-income immigrants.

Perhaps there's a U-curve. When I worked in a hotel in a metropolitan city, almost all workers there were multilingual. People from lower-income countries tended to do the unskilled work, whereas people with higher education tended to work at the front desk and in office roles, both having a mix of temporary and permanent residents.

It makes sense to me that the great in-between has less incentive to learn another language, if they're in a monolingual milieu. They don't have the economic push to find better opportunities elsewhere nor the means (or at least the idea) to travel and explore.

Depends on the population they drew the study sample from.