How to connect sensors to PCB in bulk?
The answer more-or-less hinges upon what a "significant number" of devices is. 

If the sensors you're using are commercially-available and you truly are making a significant number of devices, then the sensor manufacturer or vendor is likely willing to discuss selling you the sensor modules with pre-attached wiring harnesses and connectors to your specification (likely at an increased cost.) This is something to  work out before getting the boards made because it will likely drive your layout and part selection based on the manufacturer's or vendor's capabilities. 

The sensor you mentioned, the DS18B20, is a discrete part and I would be surprised if Maxim were willing to sell them pre-attached to a harness/connector, but it's not something I'v tried with them previously. That said, making a simple interface PCB that is not much larger than the size of one of your sensors plus the connector of interest would be quite easy and at the scale you're talking about not much more expensive. 

If the sensor is something you've designed yourself, the quantity of boards to be manufactured is too low for the manufacturer/vendor to assemble a custom harness (or they just can't/won't), then your best bet is sticking to a connector schema that's already readily available in required length and conductor number/thickness such as CAT-5 cables, EIA-485, etc., so that you don't have to assemble them yourself. Interfacing this cable to your sensor module is probably going to increase part cost and complexity, but if you don't have the manufacturing infrastructure to assemble the boards as you've shown in your prototype, then it may be the best and only option. The sensor you're using is a common one among hobbyists and as such, there are some good examples of cable interfaces to be found such as this one. The author of that page cites standard 4-wire flat telephone cable and shielded 2-wire microphone cable as potential options (in addition to CAT-5). Personally, I would avoid the microphone cable because although the shield is nice for signal integrity, microphone cables typically come with bulky pre-attached connectors because of their intended application.

If none of these are sufficient and you decide you want a custom cable for your application, then st2000 does bring up the option of working with a custom cable manufacturer, of which a quick google search turns up quite a few.