Under normal circumstances, this might save money. Want to stay an extra day or two? Rebook without paying a change fee of several hundred dollars (typical for an international flight). On the scale of a few days (except right before a holiday), the fare is not likely to rise much.
Right now, people are buying tickets at depressed prices but facing the risk of having to postpone travel for many months. By the time normal travel becomes possible, fares will rise due to rising demand. The fare increases that passengers will have to pay to use their tickets might well outweigh the former change fee. Whether the airlines say "change fee" or "fare increase", the amount is entirely at their discretion.
For a past post I looked up one country's regulation on hotel vouchers during the pandemic. The beauty of that regulation was that it forbade the provider from charging a higher price when the voucher was redeemed.
A convincing offer from the airlines would be: no change fee and no fare difference, i.e., the money you pay today covers a flight from Point A to Point B, whenever it becomes possible for you to make the trip.