For all we know a new media format will come along and make it possible to get NVME speeds cheaply, something like that would make cartridges a benefit (no installs, etc.).
The day a console manufacturer goes all digital is the day I no longer support that platform. I'm fine if they don't want my money; my backlog is such that I can't finish it in this lifetime anyway.
BC was something consoles adopted to fight the dry spell at launch. With digital stores such spell doesn't exist anymore, so there's less motivation for BC to exist at a physical level. And yeah, if I want to play my Famicom or SNES games I use my old consoles, if you collect physical copies you also collect the consoles that played them I guess.