It is indeed premature to start training. I myself started training during a downturn, and turned out to be very good timing; However that was sheer luck and naivety from a young man. Many of my classmates never found a job and were unable to maintain ratings etc. They have worked a decade as ground staff in the airport trying to pay off the loans. Now likely laid off too.
The majority of airlines won't be rehiring fresh candidates for many years. It'll be a slow callback of furloughed workforce and cautious adding of routes followed by a period of contract squeezing and ever higher flight hours before Airlines reach breaking point and are forced to hire.
The only exception I see are the Middle-East airlines who made such dramatic terminations to match the grounded aircraft, and therefore will be forced to rehire as soon as the aircraft start flying again.