I wish I could upvote this more. 95+% of startups will never need to scale past one reasonably sized single-instance database. Maybe they'll want to have a couple of replicas running for a fast failover, but from a performance perspective, they should spend their time writing efficient queries instead of horizontally scaling.

It's good to think about the what-ifs along the way. Can we shard this setup if we land a huge customer, or one that's hyper-sensitive about having their data separated? If/when that time comes, you'll have ideas of what to do about it. But realistically, most companies never hit that point. Every hour spent worrying how to make their product FAANG-scale is an hour spent not making their product.