Wow, really? It is actually super popular. Some people release routes right after they are finished, while others start releasing them after they are confident that they are far enough along with the process to ensure that there won't be too much time in between each route.
The pricing is usually based on the final price of the game. For example, if you were planning to release a game with five bachelors for $20, each route will be $4. If the person decides to buy each individual route, they are often given a code to access the unified game of they want it.
This method usually keeps people engaged long-term, keeps a constant flow of money coming in for the creator, and it allows the players to give feedback on what is available so that the creator can use it to improve other routes. I never mentioned it to you because I thought it was common knowledge amongst creators. LOL
This method may allow you to take on some of the ambitious ideas your have, but put off due to the required time commitment and lack of incoming cash.