“If it's highly relevant, users will not just tolerate but actually welcome a high volume of notifications. If you're able to increase relevance by personalization, they're much less likely to turn them off, and you can send more.”

Relevance is what users will tolerate frequency for — even welcome it

The unlock isn't 'send less' — it's 'earn the right to send more' by personalizing. Click-through rate is the proxy for relevance: when CTR is high, opt-out pressure stays low and you can scale frequency without burning the audience. That's how SoundCloud got to 500M pushes/month without nuking opt-ins.