Older vehicles (depending on the platform) often use common parts that are shared even across manufacturers. And third party manufacturers keep cranking out new stock for them.
I am hoping that this type of system develops for simple no-frills electric vehicles over time. Although laws like the one mentioned here keep piling up, increasing vehicle complexity and cost of maintenance.
Other parts were sourced from outside suppliers, which also supplied the same or similar parts to other automakers. Some of them were direct replacements, some were easily modified to work, and some could be robbed for components to refurbish your old part.
The OEM part arguably has a flaw where it dies when it gets too hot (Cadillac didn't do a recall though).
I think there were more than one third party options.