[Someone1234]

If one language is expensive, and another language is cheap which do you think employers will be building NEW software in?

I've seen people who follow the strategy you prescribe, and while they did make above market while they helped companies migrate away from [dying technology] they eventually became unemployable.

It simply isn't a good long term play, for either employee or employer.

[bargl]

It's a good endgame for an engineer. If you've got experience with X language and you're planning your exit from the industry.

Better yet, be the guy who can upgrade a system like that from X language to the newer one that can be maintained by the your new wave of engineers.

[mgamache]

In the long run we are all dead. -- John Maynard Keynes

For some, the short term payoff is the right strategy. Also how is it not good for the employer? Assuming they could fire the person (or just hire him/her as contract).

[devoply]

Why not try to do both and become an expert in the system your employer is migrating to as well while you are at it.