There's plenty of Leaf battery replacement services and parts as far as I can tell. The original Leaf is quite old now in EV-years.
With the exception that the car doesn't throw error codes and disable the power windows if I use a third-party brake pad.
We've accepted a world of throwaway phones/tablets (with a life of maybe 2-5 years). We can't afford to accept that with EVs.
I expect EVs which sell well will have robust 3rd party parts available for the kinds of parts that >95% of owners will ever need to replace, just like today. The EV industry is still extremely young, there will be much money to be made with 3rd party parts and repairs on EVs even if the automakers don't want it to be. I firmly believe that the market will find a way for the popular models.
...and that need replacing every couple years not because they don't work but because we have been disallowed from replacing parts, thus generating e-waste made up of rare earths and toxic metals and at great cost to people.
For the consumer level tractors which are manufactured in a volume which is more similar to automobiles, there appears to be plenty of parts in the market to fix them. Even the consumer level tractor parts with John Deere labels on them seem to just be rebranded cheap commodity parts much of the time.