There has never been a mandatory firmware update issued/released by DJI. It is totally up to the user whether they wish to upgrade or not.
You will not be prevented from taking off or using your Inspire if you decide not to upgrade.
Some members have said they were told they 'HAD' to upgrade or that their aircraft wouldn't take-off unless they updated their firmware - This is wrong, it has never happened.
You will be fine not updating if you do not wish to.