The upside here is that it's become extremely easy to make these kind of single-purpose hobbyist apps, and it's only going to get easier.
Yes, selling software may be dead. But instead you'll just prompt your own software for whatever niche problem you're personally solving.
The downside is that since everyone can do it, without understanding the "it" properly, security issues will be boundless and not understood, being rooted will be commonplace, and what you thought was safe and secure will be widly broadcast.
For most people, though, prompting your own software is beyond the realm, since they have day jobs to attend to, groceries to buy, children to herd, and lawns to mow, and they will be oblivious to the scams, fakes, and charlatans who have vibed up something to look useful but only aimed at getting hold of personal info and credit card details.
The future is scammy, at best.
Again with the optimistic take, but I do not think this will be an intractable problem. LLMs are becoming good at finding security vulnerabilities.
This would certainly be a radical change in how the software ecosystem operates. But I think you are ignoring the advantages of more flexible, abundant, customized software.
If anyone can make an app from a spec, how can you profitably sell an app? A million people will make their own copy tomorrow.