Following the record $5 billion antitrust fine announced today by European regulators, Google CEO Sundar Pichai penned an impassioned defense of the company’s Android mobile operating system.

“If you buy an Android phone, you’re choosing one of the world’s two most popular mobile platforms — one that has expanded the choice of phones available around the world,” Pichai wrote. “Today, the European Commission issued a competition decision against Android, and its business model.”

Pichai argues that Android offers robust competition to Apple’s iOS and iPhones and, as a result, has fostered a wide range of devices at all price points. While the EU accused Google of forcing handset makers to abide by strict rules that include pre-installation of the company’s apps, Google said some rules were necessary to ensure that app developers could write code that works across a wide range of devices.

“To be successful, open source platforms have to painstakingly balance the needs of everyone that uses them,” Pichai wrote. “History shows that without rules around baseline compatibility, open source platforms fragment, which hurts users, developers, and phone makers. Android’s compatibility rules avoid this, and help make it an attractive long-term proposition for everyone.”

Pichai noted that all apps are easy to install and un-install. And he said in terms of the company’s Android investment, it’s perfectly reasonable for Google to ask that the phone carry some of its apps.

“This investment makes sense for us because we can offer phone makers the option of pre-loading a suite of popular Google apps (such as Search, Chrome, Play, Maps, and Gmail), some of which generate revenue for us, and all of which help ensure the phone ‘just works’, right out of the box,” he wrote. “Phone makers don’t have to include our services, and they’re also free to pre-install competing apps alongside ours. This means that we earn revenue only if our apps are installed, and if people choose to use our apps instead of the rival apps.”

Naturally, the EU says it heard all of these arguments but rejected them. Still, Pichai said the decision threatens the financial underpinnings of Android, and so the company plans to keep fighting

“Today’s decision rejects the business model that supports Android, which has created more choice for everyone, not less,” he wrote. “We intend to appeal.”