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: 
There are some risks associated with it.

It can take a significant time to issue a voucher depending on airline. So, you may need to wait much time to get the voucher and pay for the flight again with it. (My Scoot ticket voucher is currently 2+ months waiting to be issued.)

The voucher also may have some conditions applied, such as it can only be valid after a specific date.

There is a (not so rare) chance that the airline company goes bankrupt and the voucher you have is of no value. Since you completed the transaction by getting issued a voucher, you cannot contact your credit card company and file a dispute.

Clarify everything before cancelling, else, depending on how much closer is your travel, you can end up paying more.
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.