Use existing old carpet as drop sheet
The theory is, splash a new coat of paint on the wall then rip up the old carpet and replace it with new carpet or polish the floor.
Using the existing carpet as your drop sheet is a good idea, only if the entire renovation is about replacing carpet and a coat of new paint, sure, use the carpet as drop sheet.
However, if you are going to do more than just paint the wall and carpet, use a drop sheet.
We use the drop sheet as a rubbish collector. We removed picture rails, stripped off paint, sand the wall back to plaster, patch all joints and holes.
The lesson we learn was, you rather than having a drop sheet to do the rubbish collection. Otherwise, you need to clean the floor, vacuum before you can step back in on the day after. It took much longer than laying down a piece of drop sheet!
Not a bad idea and it saves money.