Nothing insurmountable. You need a receiver that supports RTK. Typically the hardware receiver can do it but you have pay to unlock the functionality (typically about 3000 USD per device). You need an RTK base station (or one you can use that's not too far away) and a way to get the correction signal (local radio system or over Internet with cellular data).

With DGPS corrections, like you get with the satellite that failed, you get about 10 cm positional accuracy. For dryland crop farming, that's often good enough. It is more accurate than what most human operators can achieve and reduces operator fatigue. So many ag GPS systems are setup to use it out of the box.

There are other solutions, in addition to RTK. Trimble has "CenterPoint". There are Omnistar corrections (different sats, better accuracy than DGPS). Novatel has "GLIDE" but I've never seen it in use.

John Deere appears to sell an RTK system.

https://www.deere.com/en/technology-products/precision-ag-te...

Certain manufacturers fighting against right-to-repair certainly stymies potential third-party hacks / upgrades, such as your suggestion.

My guess: the tractors are made to support one system - so it’s technically possible but not actually possible due to lock in.