And?
PS4 was a great machine for Japan in that time, so maybe explain why japanese folks are still buying PS5, but not software. The key point was, the gap between PS5 and PS3 with are two machines that have BC, PS4 don't have BC. So, making a machine that have BC with PS4 can be a buy factor for any consumer that have a vast PS4 library.
You guys just need to use some little logic. PS4 did well on Japan, japanese gamers still had their library, physical or digital don't matter. If the Playstation 5 hardware sales are up while PS5 software are down, so people are using their PS5 for playing games they already had or using PS5 for something else.
"And?"
Ahem as the second sentence in my post you've quoted: "Such factor never inhibited the purchase of new software."
I guess you aren't familiar with the argument at hand.
Was the NDS, a console that sits among the ones with the highest total software sales in Japan history, hindered by having BC with GBA which was another successful console?
EDIT:
I add that even in the case we didn't have historical examples that shows that BC in itself doesn't hinder software sales, it is possible to come to the same conclusion by using logic.
If we assume it exists a real case of a current console' software sales being awfully low (in a reasonably long stretch of time) due to people being just content to play years old games through BC then the culprit can't be identified with BC but the blame should be squarely put onto the new games that fails to grab the attention of a large portion of the active install base.
Around the mid '00s japanese consumers didn't fail to show up when the DS episodes of Mario Kart and Pokemon appeared just because they could play the GBA episodes of Mario Kart and Pokemon on their shiny new consoles, nor the remake of Final Fantasy III for NDS was shunned cause people was still playing with the GBA remakes of Final Fantasy I + II (or the GBA port of FFIV and FFV).
Even better new experiences like Nintendogs and Brain Age were huge sensations.