Switching RS-485 bus
Let's get some things straight first, to avoid miscommunication.

1) There is no "negative voltage line" in RS-485 specification. All the negative voltages in various guides and documentations refer to differential output of the line driver. The lines themselves (usually named A, B or Y, Z) swing between ground and positive voltage (standard +5V, but low-voltage RS-485 becoming more common).

Having said that, the wide CM range of RS-485 technically makes it possible for both lines to swing even with negative levels, e.g. -2..-4V. It is your responsibility to provide your nodes with common ground to avoid situations like this.

2) The maximum number of nodes usually limited by bus capacitance, however in case of RS-485 it is limited by combined impedance of receivers long before line capacitance takes its toll. Usually quoted maximum number is 30, which means your 120-nodes row might have to be split into 4 segments. I am assuming these are your "columns". However you can find transceivers with high input resistance that allow much higher number of nodes on the bus. The LTC2872 for example has 125k resistance, for up to 256 nodes. MAX487 that you've mentioned only allows 128 nodes. Note, that this covers 120-nodes row quite nicely, so you don't actually need "columns".

3) Dealing with this many nodes involves both hardware and software. Since RS-485 is electrical interface specification it says nothing about protocol. Therefore, if you resolve the impedance issue in hardware the software task will be simplified to providing correct addressing mechanism. Alternatively your software can treat part of the node address as MUX control and select correct node cluster before beginning actual communication. In both cases, you do not need those extra MCUs connected to switches.

4) Regardless of what you use to disconnect bus segments, simple termination will most likely be not enough. Biasing resistors should be used to ensure correct idle state of the disconnected bus segment.

5) On the network that big managing half-duplex communication can be quite challenging. Most solutions will likely reduce bus through-output. I would suggest using full-duplex wiring even if you don't need actual full-duplex communication. Uni-directional lines are much easier to multiplex and/or amplify.

Keeping all the above in mind, the actual solution can be as simple as adding a MUX for Enable signal connected to an array of RS-485 transceivers, one transceiver per bus segment. The Tx pins all connected together, the Rx pins need OR logic (either wired or gate). Also you might be able to find transceivers with 3-stated RX outputs or use MUX to connect one Rx at a time.

There are plenty of the suitable chips on the market, like LTC2872, LTC1335 etc. If you choose to split the lines for full-duplex, you can use  quad MAX3030E transmitters. For the bus selector you can use any 1-channel MUX, like CD74HC4067, ADG731 etc.

So, the proposed configuration is 10 sub-nets of 120 MAX487/MAX489 nodes, plus 10 additional transceivers connected to master with two CD74HC4067 MUX chips on enable and Rx pins. No additional MCUs necessary.

If you are looking for ready-made solution, there are some RS-485 repeaters available, but they are quite expensive and you need at least 10 of them. IBS485, 8TMUX, PRO-2200. The only benefit of using these is that your software can focus on protocol only (see #3 above).

UPDATE

There is an option to actually use the wiring exactly as in your schematics, but without expensive ready-made repeaters. You can replace all those "switch + MCU" pairs in the drawing with 20 LTC2872 chips configured for half-duplex mode (compatible with your MAX487 chips) and connected with Y1-Z1 to main bus, Y2-Z2 to row segment. Then you cross-wire DY1-RA2, DY2-RA1 and you pretty much have low-cost repeaters. Here how it is done with two transceivers, but you only need one dual transceiver. And if you have used full-duplex wiring then you wouldn't even need that switching circuit in the middle.

So, all your nodes will be on the same bus. The rest is just a matter of a protocol, which in your case (single master) does not even have to worry about collisions.