Why is the first turbine in a turboprop (or turboshaft) connected to the compressor?
You can only use for shaft horse power what is available after the energy to drive the compressor is subtracted. This subtraction is done by the high pressure turbine. Any energy left in the exit flow after that can be extracted to drive a propeller, a fan or a generator.

What happens in your arrangement if the power turbine extracts too much energy to keep the core engine running? No, clearly the priority has to go to keeping the core engine alive and to only use what is left.