With no block subsidy, if the amount of txs drops 10x, then there will be 10x less rewards available for miners (actually even less if you go from (nearly) full blocks to never full blocks) and so mining must decrease 10x to remain profitable.
That, provided that parameters such as bitcoin price, energy price, ASIC price, state intervention etc., all remain constant. But they don't. If you check on the numbers, you'll see that there are times when the price fell by more than 50%, and difficulty rose more than 100%.
Of course I'm not talking about individual blocks, but about longer term trends.
Maybe there is less room for error if you consider long term trends, and exclude off-chain activity, but I'm yet to acknowledge the whole point then.
Nobody is going to mine at a loss
You don't know where the price goes. You might have purchased an ASIC, done the logistics, but there's definitely one variable you can't be sure of.