Considering they’re a huge market for Apple (~70%), I doubt they’ll pull out. And if they do, then too bad.
They won’t. Even back in the Steve days, they paid special attention to the Japanese market. The OG Macs were some of the first computers to have well-rendered fonts for CJK. Knowing Japanese culture, they will either do nothing with this new sideloading capability or they will run with it and an ecosystem will explode of alt app markets. I’m leaning toward the former.
What does that ~70% refer to? Japan is about 5% of Apple’s global revenue and iOS is installed on around 50% of Japanese phones.