Half Empty or Half Full

Why is the U. S. unemployment rate 3.7%? The Economist has their explanation:

Joblessness in America has fallen steadily thanks to the longevity and the sturdiness of the American recovery. America’s stretch of unbroken growth, which began in June of 2009, will become the longest on record in 2019 (assuming it continues).

while I think an explanation that comports better with the facts is that the wages for the jobs that are on offer are inadequate to attract people back into the job market, something explained by two-job families, non-portability of support systems, and a variety of other factors.

I guess it’s a question of whether you see the glass as half empty or half full.

I am left with a host of questions. Why do economists cling to the Phillips Curve when its predictive ability is now in question? What would the U. S. economy look like if we were calculating its size based on number of workers times the average productivity per worker rather than the way we are now? (or even better based on marginal productivity) In that case how would we calculate productivity? Is the calculation of the unemployment rate broken because the birth/death ratio (the fudge factor that has been accounting for more change in the estimated unemployment rate than actual measurements for years) is broken and the whole thing is made up? What happens to the unemployment rate when you take illegal immigrants into account?