While there is a likelyhood that there will not be a successor to the i.MX8, I think it’s unlikely that they will (or even can, due to contracts?) cancel a chip they just publicly announced. In a few months (if not already), Purism should have a frame contract that guarantees them the availability of the chip.
So, for the Librem 5 v2 (or …v6) they might have to look for alternatives. But they might want to do that anyway. And the more Librem 5 are sold until then, the more leverage Purism has to get a chip that is even more aligned to their believes. Therefore, I think the most important thing in that regard is: How many units can be ordered / sold?
Currently, 3260 are ordered, +2 every day (very linear during the past 3 months). I expect that rate will go up when we get closer to delivery and they can order 5000+ devices. 10,000 would be awesome 
The closer we get to 6-digit numbers (over time), the less we need to worry about the future.