Hi faisal,
of course there's always a hardware solution possible, it depends however, how you want to react to such event: closing the ONb_OFF switch literally (in your case) means starting up the MCU and then enabling the power regulator. Is that what you intend to arrange for, also by this intermediate event? Or would you consider it as an operator error and neglect it, going on with the MCU power down sequence, and disable the power regulator anyway? These 2 different reactions would require different hardware circuits.
By using the a.m. software solution you can react as you like, of course. Even easily change it later on, if you want a different reaction.
BTW: manually activated switches always should get debounced -- by hard- or software -- otherwise there's a chance of malfunction. Debouncing such a switch -- independent of this being done by hard- or software -- always costs several milliseconds, so a hardware solution wouldn't be any faster than a software one.