Of course its about money, they are a company, and a company's goal is to make money, But its not just the direct money from the App Store. Apple makes most of its profits from the iPhone, and if that platform all the sudden became less secure, then Apple could lose more money from lost iPhone sales than lost AppStore sales.They have arguments that support their decision, but make no mistake - the only reason they care is because of Revenue.
If apps can be side-loaded, then they can circumvent AppStore fees, including In-App Purchase fees, which account for roughly 20% of their revenue. However, that 20% of revenue has a high profit margin since its digital goods and not physical hardware. I would wager that it's the biggest cash cow Apple has today. They will do anything to keep that system closed. It has 0% to do with security. The only way this will happen is court order, but that would be an overstep for the courts.
If this wasn't about money, they would easily allow side-loading how its done on Mac OS today, and how Android does it - by default on the approved store, with the option to allow side-loading in security settings.
There is an argument to be made, that if Apple could continue to have "The most secure device" and allow side loading, they could sell more iPhones, to make up the difference in lost AppStore sales. Because most people would continue to buy from the AppStore, but a few Android users might switch.