WiFi 5 vs WiFi 6 with majority WiFi 5 devices

Well if you started supporing AX on your devices then to take advantage of AX wifi, you would need a wifi 6 system so if you wanted to support AX for your devices, Orbi AX would be recommended. 

WiFi 6 has higher connection rates than Orbi AC, I don't see how Orbi AC would be better than Orbi AX. Also you need AX supporting devices to take advantage of Orbi AX high speed rates. 

The main difference is AX4200 is 600 + 1200 + 2400 and the AX6000 is 1200 + 2400 + 2400.

That's a 2.4GHz connection rate of 600Mbps vs 1200Mbps, but you might not be using 2.4GHz anyway, since multiple sources of 5GHz are usually better than one long range, congested 2.4GHz connection.

Then the local 5 GHz connection bandwidth available doubles from 1200Mbps to 2400Mbps, and finally the 2400Mbps for the wired backhaul remains the same.

Additionally, the AX6000 has support for 2.5Gbps ethernet from the WAN whereas AX4200 maxes at 1Gbps.

Please review the spec sheets between the Orbi AC and AX models to compare speeds.