So it's now going to be possible to use magsafe for non Apple device but that are using USB-C charging?

No. Magsafe is a different port. The magsafe cable is magsafe on one end, usb-c on the other. The magsafe end being the bit that pushes power out.

It's a USB-C brick with a USB-C to MagSafe cable. You can remove the magsafe part and use it with a C-C charger

And the MagSafe cord retails for $50.

For a power cable.

I wonder if you can still use the old magsafe power cords to charge the new machines, even if at a lower wattage. Those charger cables aren't cheap, and I still have a few from old MBPs.

You can keep using them with a Magsafe-to-USB-C adapter, for example from Elecjet [1]. I have a couple of those and they have worked just fine with my 2017 MBP.

[1]: https://elecjet.com/products/anywatt-magsafe

I highly doubt it, MagSafe 1 to 2 looks similar but are different physical dimensions. MagSafe 3 looks to be the same, the port looks significantly skinnier/shorter than 2 (about the same as the usb-c ports).

I'm guessing its not physically compatible, since they made a point of calling it MagSafe 3. But we won't know until a machine lands in the wild.

It is shaped differently and they said it was updated to support more current.