USB Type C Power Delivery with STUSB4500, making the device compatible to USB 2.0/3.0 using a USB A to USB C cable
what would you do to make a high power device using USB Type C power
  delivery compatible to an 'old' USB A only computer


High-power bus-powered USB Type-C device is INCOMPATIBLE with regular USB-A ports, period. So the standard solution is to provide an external 5V 2A AC-DC power adapter.

As a user-friendly feature, all you can do is to provide a non-standard TypeC-TypeA cable (with 22k pull-up) with your device, and pray that the Type-A port on user side can handle 1.5A load without significant drop of voltage. A a safety, you can implement voltage drop control on your side and disengage if the host port is too weak for 1.5A.  


  AND a 'plain' USB A charger?


Plain USB-A chargers most frequently have DCP signature (D+ D- shorted), so implementing a D+/D- short check will provide you with desired power. I believe there are C-C controller ICs that can do the BC1.2 and DCP check.

And since you are fine with standard 5V 1.5A power, you don't need any PD complexity. The standard Type-C Rp/Rd signaling logic will do the job without any Power Delivery contract negotiations.