I was thinking the only option that makes economic sense for Nisus writer is a subscription.

If the market is saturated and they're not going to sell any new copies, then they're just going to go out of business.

If they have existing customers which want the software to continue to exist on an operating system designed to make existing software stop working every few years... then customers paying for the privilege of keeping the thing working on Mac OS seems like the only option.

(For reference, on Windows you can just run stuff from 1995 with basically no problems.)

I hate subscriptions as much as the next guy, but if something is mission critical and irreplaceable, the $15/month for "I need this to keep working" seems pretty reasonable. If there's a non-trivial number of people in a similar situation, maybe they could work something out.