OP's question is far from stupid. It's also not a "dirty little secret" as the OP suggests. It's a known issue that has been discussed since Bitcoin's early days.
The fact that 2140 is a long time from now doesn't make the issue go away. A good project should be sustainable and not have an expiry day, so it would be nice to have at least some idea how things could work when block subsidies become insignificant. Discussion should be encouraged, not stifled.

The original idea was for the transaction fees to become significant enough to replace block rewards, but we now know this may not work. I guess, in the worst case scenario, we could have big players (i.e. exchanges, BTC-related fund managers etc) to take responsibility for mining, or we could change the consensus algorithm to something else like PoS.

And it's not like we can just sit back and relax till 2140. i.e. in less than 20 years, block reward will fall below 0.1btc. Depending on the price, this could become a security risk sooner than many think.