A forced spare parts system, if one existed, would encourage manufactures to use off-the-shelf parts wherever available. If no off-the-shelf part currently exists, they would want to get a company that specializes in those type of parts to add one to their publicly available range. Not necessarily a stocked range, but something that could be ordered by anyone. (I'm thinking about generic things here, pulleys, belts, wheels, generic gate ICs etc, not ASICs or anything IP related)
If there's a market for a part someone will buy 5000 from that manufacturer, or whatever the min order run is, and then resell them.
It means that the manufacture has only a very small number of spare parts they themselves need to supply.
It's only really things like ASICs or other stuff that has IP or for which blocking competitors from using it is desirable.
I think we can all agree that using a standard or protocol is better than inventing your own one in 99% of cases.
It should be the same for parts.