What if transaction fees aren't enough to incentivize the miners to secure the bitcoin network? I think this is a potential problem we may see in the next few decades as the halving continues on.
It's likely miner with less efficient ASIC or more expensive operational cost (e.g. cooling and electricity fee) will shutdown their activity. And after next difficulty adjustment, other miner (who have more efficient ASIC and cheaper expensive operational cost) could continue mining not at loss. It's also possible some miner continue mining at loss (at current exchange rate), but speculating BTC price will rise so they can turn loss into profit.
However, for how long can they continue? Will they loan some money to use for their cashflow? It will certainly not be sustainable if there are no assurances of a stable reward at every block mined. Asic manufacturers might also begin to think twice about their own business because of the insufficient assurances on the profitability of their customers.