In the real world (non-photography), I sell what the customer wants to buy.
Who is your customer? (A publisher? An advertiser? A stock photo agency? A real person who will hang it on their wall?...) What are they asking for? Given the flexibility we now have, I don't think it is up to you to limit your customer.
If your customers want to buy a digital image, I'd guess it is because they need the flexibility that digital provides in the modern world - it fits into workflows, it has an effectively infinite storage life and lossless copying of the original if managed correctly, it can be repurposed with ease.
If your customer wants to buy something to hang on the wall (or to hand out or some other physical "photo"), that's what you should be selling them.
If your customer wants to be able to come back to you in 30 years time, it's up to you to store the originals properly - negative, print, digital file or whatever - using the appropriate technology, and filed in an appropriate manner. If you want your estate to be able to find them, it is up to you to index them and file them appropriately. (I'd personally charge if someone wants me to store an image for them.)
If it is a negative, or a print, or a digital image, unless filed correctly with identifying details and some indexing method that a third party can follow, it might as well be a shoebox full of holiday snaps when someone else goes to look at it. Your model friend's photo would never have made it back to her after the photographers death unless a) someone in the family recognised her or b) it was properly filed and indexed. It doesn't really matter if it is a shoebox or a hard drive.
I find it very hard to see what the problem is here. Digital files are just another form of negative - you just can't pick it up.