Welcome to the forum @Davidd
Try asking that of even a company, and see what they say. Also see other topics here.
Even huge companies that seem very profitable may seek profit by killing off products.
Google is notorious for this. The ending of CrashPlan Home backup sent users here…
Nobody can guarantee the future, but 2023 Duplicati planning can be followed at topic:
2023 release planning
where it looks like there are finally enough volunteers to put out a long-awaited release.
If Duplicati ever becomes unavailable for future backups, you can still restore without it:
Independent restore program
and if Python also dies (seems less likely), I think there are some Rust versions around.
How long is “a long time”? In computing, things change. U.S. Library of Congress notes:
Digital Collections Management Compendium
One point that I always stress is that Duplicati is for backups, not for long-term archives.