The other scenario where I would find "poaching" somewhat unethical is when the primary purpose is depriving a competing company of talent.
In that scenario, in order to entice them you might offer bonuses and salary that are disproportionate to the value those employees would contribute to your company, but the loss of those employees would cripple the competitor (loss of institutional knowledge, schedule delays and cost of finding replacements, morale, etc).
The company can counter-offer but that costs them money they wouldn't otherwise have to spend, to protect their "investment" -- the sum of the parts as you mentioned. Ideally a company would be resilient against the loss of employees (e.g. getting hit by a bus) but smaller companies like startups don't often have that luxury, which makes them more vulnerable to poaching and other shenanigans instigated by large companies.
In that scenario, in order to entice them you might offer bonuses and salary that are disproportionate to the value those employees would contribute to your company, but the loss of those employees would cripple the competitor (loss of institutional knowledge, schedule delays and cost of finding replacements, morale, etc).
The company can counter-offer but that costs them money they wouldn't otherwise have to spend, to protect their "investment" -- the sum of the parts as you mentioned. Ideally a company would be resilient against the loss of employees (e.g. getting hit by a bus) but smaller companies like startups don't often have that luxury, which makes them more vulnerable to poaching and other shenanigans instigated by large companies.