The idea of a "passive investment" is a joke.
The example of a web app as a passive investment is crazy -- how many apps exist that won't require support, ongoing maintenance, etc? That's significant work.
Stocks, bonds, mutual funds -- the returns here are small enough that you can't generate income to live on without a huge up-front investment. If you could manage a consistent 8% yield you'd be doing well, and it would $1M to generate an $80k annual income.
Real estate -- I know a couple of part-time landlords, and this is not passive work. They have to deal with tenants, maintenance, etc. Sure, you can hire a property manager, but that cuts into your returns.
Royalties -- This is hardly passive. What's the half-life of IP these days? You can write a book, but it will likely be out of print within the year. If you manage to publish something reasonably popular, you'll likely have to build a franchise around it, which means that you'll have to work at writing more books. Even if you hire ghost writers, you have to find them, manage them, deal with marketing, etc.
One of the takeaway lessons I got from b-school was that "there are no long term economic rents". In other words: if something is profitable (easy money), then other people will enter the market and the profitability will decline. Sure, there are barriers to entry, etc. but all else equal the crowd will follow the easy money and suck the profit out of it.