More than becoming difficult - it is difficult for certain parts. While the labour cost of servicing a 321 and 861 is the same, the overall cost can be higher if parts are needed, because if the part you need is discontinued, you have to rely on the open market. On the open market, people who have rare parts usually know that fact, and they charge accordingly.
I have paid what seem like stupid prices for parts that are needed, based on the complexity of the actual part - it's the scarcity of parts that makes them valuable.
Cheers, Al
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