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.
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.