You are fundamentally misunderstanding how this entire process works.
SP has a limited capacity, they can only produce so much at a time. I believe they have a single manufacturing line for SA key caps. The lead time is literally how long it will take to produce all the other in progress group buys and custom orders to GET TO your caps. The only way to produce carbon faster, would be to jump everything else in line, but that just means you delay something else.
I don't expect Dasher/Dancer ppl would be too happy if you told them you delayed their set, nor would people who ordered SA strong spirit, or people who ordered SA 1965 or SA Ocean Dolch or SA Senna or SA 88 Commando or SA Photo Studio or SA Blue Mods or SA Cream Cheese & Green or SA Corner Creatures.
That's at least 10 other open group buys for SP SA Double-Shots. How would you explain to all of them why they were ALL delayed because some people couldn't deal with the wait on carbon.
It's not that they CAN'T produce any given set faster, it's that people are literally running group buys and ordering more custom key caps than they have the CAPACITY to produce, so even with constant production, there is a queue.
So yes, sure SP COULD add another line, but I would imagine 9 months lead time isn't even close enough to justify the cost of expansion. You have to keep in mind, an expansion only works if it is viable LONG TERM. The cost of equipment for a second line and the extra costs associated with running it would be TREMENDOUS up front. They would need new machines, more space, etc. I bet it would be in the hundreds of thousands of dollars. So then you gotta think, what would the end result be?
The lead time is slowly growing for sure, but will it last? How long? Will the rate increase or decrease? As it stands right now, I bet if they added a second SA doubleshot line, they could start reducing the lead time, which is great for us, but what would the benefit be to SP? Reducing lead time sounds great in theory, until you realize what it actually means. Reducing lead time means you are operating under capacity, and that is bad, as that means, once you have killed your queue your expensive investment is just sitting there idle.
The point being no company is going to invest in increasing capacity unless they are also expecting to increase volume to match. We are talking about spending what would likely be more than the net profit they will make on the entire SA Carbon order. It just doesn't make sense to blow that much money unless the end result is doubling sales, which seems ambitious and risky.
I bet if in another year or two or three the queue has continued to grow and orders are still rolling in, then they MIGHT consider a second keycap line, but for now I imagine it's extremely unlikely.