The price will go up because DDR4 won't have any new manufacturing and all of those DDR4 system owners will be looking to upgrade their systems instead of moving to DDR5. So for a period, demand will go up but there won't be any new stock so existing prices will increase.
Because it's cheaper to just add more memory instead of memory + processor + motherboard. And the performance of a DDR4 system will remain usable for a very long time, probably another 5 to 10 years.
This happened with both DDR3 and DDR2, their pricing went up when the next technology became mainsteam.
The prices will go back down when all of the branded stuff is completely sold out and only refurbished/repaired modules are available in the market. That's what's happening with DDR3 now, everything being sold is just a new sticker on old chips and modules.