Don't think GTA will ever surpass the Greater Los Angeles area in population or GDP.
It will probably pass Chicagoland in GDP and population by 2030 to become the 3rd most powerful city in North America excluding Mexico City.
Toronto closing the gap with Los Angeles by population is quite plausible. If 2018-2019 population growth numbers stayed constant Greater Toronto-Hamilton would pass Los Angeles MSA in population by 2050. I'll use Greater Toronto-Hamilton (Toronto CMA + Hamilton CMA + Oshawa CMA) as it's the best catchment area to approximate a US MSA imo. Extrapolating out to 2050 is, of course, problematic as things rarely stay constant but it indicates what's possible and when.
Below is the population of each with the absolute change over the previous year in brackets. The statistical analysis shows that Los Angeles is very catchable and by mid-century. Los Angeles MSA is bordered by another 4,650,631 (2019) in Riverside MSA while Greater Toronto-Hamilton is bordered by roughly 2 million in the rest of the Greater Golden Horseshoe. Nonetheless, I suspect in 20 years people will be comparing Toronto to Los Angeles rather than Chicago. Greater Toronto-Hamilton could reel in New York MSA too although that would take a few decades longer.
Los Angeles MSA in 2019: 13,214,799 (-35,080)
Greater Toronto-Hamilton in 2019: 7,680,502 (+144,566)
-35,080 X 31 years = -1,087,480
+144,566 X 31 years = +4,481,546
Los Angeles MSA in 2050: 13,214,799 - 1,087,480 =
12,127,319
Greater Toronto-Hamilton in 2050: 7,680,502 + 4,481,546 =
12,162,048
This page features Vintage 2019 population estimates totals and components of change.
www.census.gov
Annual population estimates as of July 1st, by census metropolitan area and census agglomeration, single year of age, five-year age group and sex, based on the Standard Geographical Classification (SGC) 2016.
www150.statcan.gc.ca