had not expected such a basic mapping to not come as default... what is the simplest way to do that simple mapping - that does not require modifying existing scripts?
The whole idea of the new input system is to stop using the legacy
Input class entirely. It's a full replacement, not just a few extra features.
If you want legacy scripts relying on
Input to keep working unchanged, you can run both input systems under
Project Settings > Player > Active Input Handling, as shown in the documentation:
This means you're asking your Unity game to do twice as much work piping input through two different systems every frame. It's OK during early migration and testing, but I'd strongly recommend biting the bullet and editing the old scripts so everything in your project uses the same input system.