One problem I see is that the feeder for EF lens development is full frame bodies. If we are at the end of the line for full frame DSLRs then we will inevitably be at the end of the line for EF lens development. A self-fulfilling spiral of death.
Sure, Canon could continue to develop EF-S lenses, but they showed little interest in EF-S lenses even during the height of the DSLR era. As long as Rebels are selling well, they might make some changes to EF-S lenses to reduce production costs and incorporate other efficiencies, but I don't see them developing new EF-S or EF lenses.
In my opinion the current level of sales for low-end DSLRs is not particularly illuminating and constantly falling back on those sales figures doesn't really prove anything. They are a snapshot in time, but can't predict the future.
Cell phone cameras will keep getting better and keep cutting into the Rebel market. The market will keep aging out and new young buyers won't be sufficient to replace the older ones. The younger buyers will focus on documenting their experiences through videos, selfies and food photos (all better suited to cell phones), rather than collecting a catalogue of the things and people that were more suitable for traditional cameras.
I don't envy any camera company, trying to stay in business by selling increasingly expensive products to a shrinking and aging market, with no good prospects to replace lost revenue by attracting new customers.