Sales-wise, the most significant thing about BC isn't even really letting people keep playing games they bought 10 years ago, it's opening up access to huge back catalogs of games to sell without having to go through the trouble of remastering or remaking all of them.
GOG/Virtual Console/PS1 Classics are/were entire business models built off of this. From what I understand publishers love that extra bit of revenue they get from putting 15-year-old games on sale on Steam periodically. Sure, Sony and Nintendo get that in a selective sort of way by simply remastering and re-releasing a bunch of old games, but simply allowing digital BC is the only way the majority of those old libraries will ever become available. PS1/PS2 Classics allowed Sony to pretty much just toss ISOs onto PSN after getting the licensing worked out. Were the licensing costs so great that Sony simply decided to remaster and re-release those games for $40 each? Or $15 in the case of the emulated PS2 games on PS4?
Again, this may become a bigger issue in the future as today's games-as-a-service titles become legacy games in the age of the PS5 and Next Xbox, and maybe Sony and Microsoft will want to support some gamers who continue to play those games and buy lootboxes for them.
