In most companies a good engineer overgrows his position after few years. There’s no economical value of holding him back. It’s cheaper to hire a replacement

In most companies a good engineer overgrows his position after few years. There’s no economical value of holding him back. It’s cheaper to hire a replacement within planned budget. Another point — in well organized business any engineer in any position is a replaceable unit. Otherwise business is not scalable. So companies do the right thing. The right question is — why many engineers are staying for so long, even though its not profitable for them after some time/experience?