Correct, past R&D spend is already sunk and can’t be undone. But that’s why it’s useful to separate sunk costs from future operating costs when evaluating viability.

The relevant question is whether the ongoing revenue from the existing product is strong enough to support a sustainable level of R&D going forward. If your runtime margins are healthy, you have options: scale back R&D burn, focus on incremental improvements, or use the profits to fund more ambitious projects.