The biggest cost of producing parts is making the tooling to mass produce them. Manufactures will retool to produce other stuff once they have enough of a given part, but if there's a shortage (or they think there will be one soon) they'll bring the old tooling out of storage and set up the line again. It's common for auto companies to supply new parts for decades after a car is made.
Heck, Mazda makes new parts for the original Miata.[1] While the first generation Miata is a recognizable car, it's not very popular. A total of 433,000 were produced. Maybe half are still on the road today. That may sound like a lot, but twice as many Ford F-150s are sold every year. If it's profitable to keep making parts for 200,000 vehicles, it's going to be a long time before most ICE cars run into shortages.
Heck, Mazda makes new parts for the original Miata.[1] While the first generation Miata is a recognizable car, it's not very popular. A total of 433,000 were produced. Maybe half are still on the road today. That may sound like a lot, but twice as many Ford F-150s are sold every year. If it's profitable to keep making parts for 200,000 vehicles, it's going to be a long time before most ICE cars run into shortages.
1. https://news.mazdausa.com/2019-10-28-Mazda-Expands-U-S-MX-5-...