It's difficult to predict when the reward will be negligible.

In 10 years the subsidy will be 3.125 BTC per block.
At the current exchange rate, if the miner collects no transaction fees at all, that's nearly $8600 per block.  Is that "negligible"?

In 11 years the subsidy will drop to 1.5625 BTC per block.
At the current exchange rate, if the miner collects no transaction fees at all, that's nearly $4300 per block.  Is that "negligible"?

Of course, there is a real possibility that the exchange rate could go up over the next decade or so.

In 15 years the subsidy will drop below 1 BTC to 0.78125 BTC per block.
If the exchange rate is $300,000 per bitcoin, then even if the miner collects no transaction fees at all, that's still $234,375 per block.  Is that ""negligible"?