The system is designed by a licensed engineering, installed by a specialty contractor, and inspected by the authority having jurisdiction.
Commercial construction is not just like your house only bigger.
A WiFi connected smoke alarm is full of incidental complexity. That makes it inherently less suited to function than the simpler design.
Building these correctly is as simple as building a dumb alarm and adding a second product in the enclosure (a radio link). If built correctly, nothing in the smart bit can sabotage the dumb bit. For example, it can have its own power supply. Or be completely isolated from the alarm.
Others in this thread have described how they put an esp8266 or similar right next to their dumb alarm and have it report when the alarm goes off by detecting the sound. This has zero additional failure modes!
If you can screw it up, or the smart features adds any risk - it’s a bad product.
I’m not arguing that no connected alarms are bad. I’m arguing that It’s possible, and easy, to make a connected alarm that is no worse than a dumb one .
That’s a completely different argument. It’s trivial to make a lousy product. Even easier with a product that tries to be smart. That doesn’t make it a fundamental law that it has to be that way.
I don’t doubt that Nest products are useless, their smart features require an internet connection which is a clear sign of trash in any home product.
Taking a dumb alarm and putting a radio or other connection on it doesn’t (shouldn’t) affect the functionality. These alarms also existed since forever for larger buildings. No one would argue that they were less reliable than dumb alarms.