After years of rumors, it's finally actually happened: RIP Sonic as of October of this year (according to the folks at the Authority). So MY 2020 is in fact the last year. I may be the only one, but I will miss it! As for a replacement:
"... there is currently no replacement planned. That said, it’s possible GM could bring the Chevrolet Onix over for the U.S. and Canadian markets following its best-selling performance in South America."
gm authority dot com/blog/2020/07/gm-confirms-chevrolet-sonic-set-to-retire-this-year/
I might pick up a 2019 stick-shift used at some point to replace my 2015. Or make the switch to something else, I'm not sure. RIP.
(As for Chevy's sedan/hatch lineup for 2021, we're down to Spark and Malibu.)