Suggestion for a power on/off system circuits
There's a chip for this job

If you're OK with a single chip solution and can work with SMT ICs, I'd use a LTC2951.  It needs a minimum of external components; namely, a couple of ceramic caps and a pull-up for the interrupt pin, as well as the pushbutton of course.  It also has a wide supply voltage range, even wider than the 4000 series CMOS used in the circuit you posted.