Question: I'm analysing the technologies for an IoT device that would need to be installed in a commercial environment. The requirement is to install thousands (or maybe even tens of thousands) of small sensors/devices in a commercial setting, like a hotel, or an office building. I need these devices to have bidirectional connectivity to a central server/control dashboard. The connection doesn't have to be super fast, but it should be reliable.

No power usage restrictions present, but I also don't want to considerably stress the existing infrastructure or spend lots for a new one.

So, what are my options? The following ones come to mind:

1. Wi-Fi. The obvious choice, but it has the following issues:
a) Too many of the devices in the same area will definitely stress the radio space, cause interference and possibly make the connections unreliable for both iot devices and the existing devices (computers, tablets, phones, printers, whatnot)
b) Might need lots of routers/repeaters to cover large areas, which may impact the cost of the project.
2. ZigBee. Has issues with WiFi in the same space, also kinda limited in number of devices? Will also require specialised hardware and bridge to IP network for control dashboard.

Mesh networks also have their limits.

I'd love to hear opinions, ideas, suggestions and resources to look into.

Answer: 
LoRaWan is clearly a good options, however take care about duty-cycle restriction that can apply depending on the Zone (US/EU/AS).
This Duty-cycle can be a difficult point to handle from Gateway perspective (and so Network planning) if you have lot of devices with Downlink (or Uplink acknowledge).

For example in EU you will have channel with 0.1, 1 and 10% limitation.
It will be interesting to have more information about size of data payload in UL/DL and period of the messages (if periodic).

Other things is that you might need multicast option in downlink.
If you need higher data rates than the already mentioned LoRaWAN can provide you can consider Bluetooth. BLE can theoretically support 32k nodes in mesh configuration or more in point-to-point (though range might be a factor then). <https://www.bluetooth.com/bluetooth-technology/topology-options#mesh>