You deploy to all instances simultaneously. A bug affects 5% of requests. Before you can react, 100% of users are hitting it. Canary deployments let you catch that bug when it's hitting 1% of traffic, not 100%.
Your system handles 1,000 users today. You're designing for 10,000. Not 10 million — 10,000. Most "design for scale" advice is written for companies you're not. What actually changes at 10x, and what's over-engineering that will hurt more than help?