In Sony's case, it really doesn't matter how "easy" it is for them. We've seen many other marketplaces for digital media (not just games, but also music, movies, and books) that have been able to continue to adapt and evolve with the times.
We're not asking Sony to make a bunch of new games for these systems. We just want them to continue to make these games available.
One of the touted advantages of digital distribution was that these games could continue to be bought and sold for years and decades to come, without regard for availability of resources to manufacture physical materials. I know that other niche game marketplaces have closed previously (as I said in my earlier post in this thread--also PS Mobile from Sony as another example), but this market has matured now compared to where it was several years ago. If Sony isn't willing to keep these games available while their current competitors are able to do so, then it will reflect poorly on their company, and cause consumers to distrust them for future digital software purchases.
Yes to most of that, although the amount of de-listed content in other fields is way more than anybody takes notice of, it's just not as apparent and concentrated as a whole console library going down. Licenses expire all the time, lawsuits of ownership put the sales viability of a product in question, and companies go under. There are notable movies and shows which got VHS or even DVD releases but they're since out of print and nowhere on streaming for one reason or another. Music/movies/books lose product pretty consistently, but the equivalent of a full console delisting doesn't happen all at once; that would be like a Warner Bros going out of business and taking its library offline until being acquired (and even that's temporary) or an HBO failing and all that content not having a future home (granted, the big-ticket items in that library would all be acquired, but you might never see Arliss or The Borgais or Dream On available again, if they even are now?) Disney's purchase of Fox has left a good number of titles in limbo (and even Disney's own transition from physical to digital media has a few questionmarks on some titles' futures,) because if they don't fit on Disney Plus, they might be Hulu fodder but it's not as easy to roll them in or monetize them and so they own the properties and can host or license them as they wish, but they can also just as easily forget they exist and see no significant difference in their bank account.
There are few cases as devastating events of closures of an online marketplace as a video game console eshop closing up because there's just not the concentration and volatility and age of value as in gaming ... but there are lots of little calamities in every other market.
Also, yes, digital distribution did come with the advantage of a longer and easier shelf life of products, but never the promise that everything will be forever.
Going digital erases the burden of carrying physical inventory, so companies can take chances and let a product ride so much easier, but eventually, it may be more hassle and distraction than it's worth to sell legacy titles for legacy platforms instead of concentrating 100% on current products. It may come a time in any product's lifespan where it costs more money in stamps to send out royalty checks for a couple purchases a year than the product actually brings in. Entertainment producers have been able to offset the problem of marketing depreciated titles by putting together packages of media (thus you'll see batches of the same couple dozen movies come and go off Netflix every month,) but that wouldn't work in video games because you can't just take your Wii library over to the PS4 market (and it wouldn't solve this problem anyway since it's not the publishers closing the PS3/Vita store, it's the manufacturer.)
And while I agree that this sucks and makes me think badly of Sony for not putting the effort in to maintain its full platform library, I'm sure they've done the math as part of this decision. Pissing off a number of GAF posters and hardcore PS collectors may still be worth it when the vast majority of PS gamers won't even notice, or will grumble for a few days but eventually forget that there was an old library to go buy. (There are only 5M Vitas in circulation, how worried can they be about making us Vita owners angry?) And as far as distrust, your digital software purchases on every platform are still right there (for now?) (aside from this supposed kernal clock bug,) you just can't go out and purchase new digital software.