I think this is too simple a rule personally. What you want to optimise for is the discounted long-term cost of not automating vs the upfront cost of automation

I think this is too simple a rule personally. What you want to optimise for is the discounted long-term cost of not automating vs the upfront cost of automation. While I'd agree developers often prematurely optimise, advocating to put off optimisation is crippling in many cases – especially if you're building a company with a small team. Being obsessive about efficiency in a startup is an unappreciated factor in what separate companies that succeed from those that don't imo. It's also not so much that it hurts to not optimise, but that after a while most your day will be filled with mundane but achievable tasks, and this will prevent you from spending your time more productively. It's the same reason you shouldn't put off fixing tech debt "until it hurts". If you wait until it starts to hurt then you've probably been suffering the efficiency / security / performance costs of putting that work off for some time. Just because it wasn't noiticably hurting doesn't mean it wasn't a cost.