There are certain costs associated with hiring someone. Such as:

1) Recruiting 2) Interviewing 3) Onboarding and introducing to team 4) Provisioning equipment, office, etc. 5) Ramping up and getting integrated

Now, if these costs are paid per employee, then the longer this employee works with us, the more valuable they become and the more money they can make.

BUT

Those employees that get poached, or "fail upwards" into other companies, or whatever, actually make that position cost more. Way more. And I am not just talking about the fact that they might leave mid-project or have a moral hazard of screwing up a project knowing they'll have their next job to fail upwards into. But something even more basic.

For example if I have to pay the cost of 1-5 once every 3 years, that's 3 times less than if I have to pay it every year. So will I bear the full cost of 1-5 every time or pass it on to the employees I hire into that position? Of course I would pass it on, and moreover I can justify it by saying that they don't have "seniority within the organization" or that we are a meritocracy.

So you are completely neglecting this side of the phenomena, and blindly claiming that salaries will go up because poaching increases. It's not at all obvious that they would!