The halvings of the past gave the same amount of bitcoin rewards with bitcoin mining, excluding mining costs, electricity bills, transaction fees, etc.  In case of next halving, the Bitcoin reward goes after the fraction i.e. almost reaches zero, in which case it is very difficult to predict how much it will be reasonable / profitable to mine with electricity bills, transaction fees, mining maintenance costs.  If the Bitcoin market goes down then mining will stop altogether.  Especially after 2040 it is only a matter of time whether Bitcoin mining will survive.