Conventional wisdom, plus a few studies, has shown that if you want to get 15% or more of a raise, you need to jump somewhere else. Since they're hiring from the market, they have to offer (decent) market rates, while your current job is going to keep paying you at whatever rate you agreed to.

After a while it becomes a self-fulling prophecy -- they know you're jumping in 2-3 years, so they're not even going to try to keep you.

The talent pool isn't shrinking, either, so there will be more devs, and more IT guys -- be they in Seattle, or India -- as things more forward. This means they can probably find someone to replace you, and while it may be a loss of productivity, the lower salary (relative to increasing yours) may offset the costs.

Plus inside threats are big -- a senior dev-ops guy who has been there for 8 years knows how to break things just right, skim things off the top, or just take a "agent fee" from some vendors to push for their product to be rolled out. Keep em under a 3-4 years and they can't get their roots in enough to damage things.

That didn't answer my question at all. You said "if you want to get a 15% or more raise, you need to jump somewhere else". That's just coming out of thin air or based on "other companies do it". If you do work in HR or as a manager, what's the rational basis deciding just that number?