> If the actual temperature at the airport is important to any set of users enough so that the difference between it being 18 or 22 deg C is relevant, one should expect that there be at least 3 sensors

Three sensors doesn't solve the problem. Manipulating becomes marginally more difficult with three sensors, but it's still very possible, and with enough monetary incentive it's still even likely that it happens again.

So why not 5 sensors? How about 10?

And what about more consequential issues like the toppling of governments or military blockades where true redundancy is impossible and people are actually harmed?

Is there not a point where you start to blame the incentives that are being chased?