Dilithium is great for doing M/AM, apparently. But is it also a vital element in a warp engine in all cases, regardless of the nature of the power source? Romulans for one seem to find great value in dilithium, despite going for those AQS power sources of theirs big time, and we don't know of any alternate market for the crystals besides warp engines.
We might think that the magic of warp is all in the coils, and if you huff and puff into those hotly enough, lo, FTL. But episodes like "Fair Trade" suggest that the plasma going to the coils is already magical in itself, rather than merely a conduit of energies. And we still don't know the nature of energy being carried by the plasma from the power event to the coils. In Fed engines, that energy comes out of the dilithium, or at least through the dilithium, which in itself is seldom claimed to be particularly energetic, and even in "Alternative Factor" needs to be stolen from a special energizing chamber in order to do good for the bad guys. But having the energy come out/through the crystals might be absolutely vital, even if it goes into the crystals from a fission pile, a coal furnace or a zillion cats being rubbed real fast.
The amount of energy might not be all that crucial or remarkable in the end. Even the makeup of the warp coils might not be that significant, and Cochrane could cobble them together from fairly ordinary materials, and Geordi and Reg then fix it with basic copper. But at some point, we need to bring in the magic, to make it all plausible. And having it all hinge on dilithium, rather than power as such, is in keeping with everything Trek tells us. Indeed, it's a big part of what has been recently told: other ways of doing FTL (annd yes, those are legion) are more difficult and costly, not less.
Timo Saloniemi