Question: I have booked a ticket for future travel but now see the price has come down considerably. Should I cancel because of coronavirus and rebook with voucher?

Answer: 
You can't just 'cancel because of coronavirus'. If the airline is still listing the flight then you can only cancel on whatever terms you agreed to when you booked.

If your ticket is fully refundable then you can cancel and rebook, if you can actually get the refund. Many airlines are refusing to pay refunds at present. 

More likely, there will be some penalty for cancelling, which could be part or all of the fare you've paid. Even with the price reduction, it's likely that you'll end up paying more overall.

If you're hoping to get a full refund (as cash or voucher) then you'll have to wait until the airline cancels the flight, in which case there will be no flight to rebook onto.
It depends on the type of your ticket you purchased. No airline will cancel your booking because of the corona. I tried that. I didn't have a non-refundable ticket (or whatever it is called). However, I had a flexible ticket so I just changed the dates of the flight.