I'm pretty sure i'm never going to find a plastics set for my Sega Apollo 13 pin.
I did find a spare blue moon ramp though.

Hmmm..
I don't see them.
Do you have a link?

Is the "license crackdown" the reason for so many unobtainable parts and lack of parts in general?

Sounds like I should ramp up my spare parts collecting. I've got 24 pins and plans to buy 15 more. All lifetime keepers.

Very true. With Stern producing around 3 title per year. Making parts for 30 more pins would be difficult. I own a Roller Coaster Tycoon(for the kids mostly). I have spare ramps and a plastics set, but have not seen any for sale in a long time.
I'd expect many titles to be like that in the future.

It can get expensive buying parts for so many machines.
Half the parts I will probably never need, but it seems prudent to buy them as I see spare anything for sale.

What spare parts are good to buy before they become unobtainable for pins that are lifetime keepers?

I actually obtained a complete NOS set of Apollo 13 plastics the other day.
Yay me!

Yes!!!
I had an easier time finding a set of Sega Apollo 13 plastics.

It's really great to see lots of parts being remade. If it wasn't for this golden age of pinball parts availability, I wouldn't have entered the hobby. Nothing worse than having thousands or hundreds of thousands of dollars of pinball machines that can't be played due to no spare parts.