A non-TTN node was sending every 6 seconds with SF12 and effectively blocking the nearest gateway. OTAA was nearly impossible.
It shouldn’t - unless your scheme only supports one frequency, or unless that node is using the same frequency selection plan you are, and you spend periods of timing being synchronized in your execution of such.
Now imagine that some nodes continiously transmit just LoRa preamble.
That would probably be illegal in most places.
First, you generally aren’t allowed to transmit narrow bandwidth signals continuously. Maybe you mean “frequently”
You’ll see no uplinks from other networks in this case, and you gateway will be effectively blocked.
Not really. One pre-amble sender would only hit one frequency and spreading factor. A gateway not close enough to be swamped outright or picking up the skirts of the signal should be able to receive on the other channels and even the other spreading factors of that channel.
Are you going to ask the GeheimStaatSpaceMarines to come and rescue you
In most places it is illegal to intentionally interfere. A system just playing “made you look!” with LoRa demodulator slices by sending preambles but never following with any data would likely be hard to defend as anything other than an intentional interferer.