That you're trying to screw them. They will then proceed to hire someone else who will happily do in a few months what you claimed would take years. It'll probably involve throwing out all your code and starting from scratch in the new programmers favorite language/framework du jour.
To extend concept to programming languages:
When you need / really want a feature and the language is failing to give it to you there's a decent chance you'll throw out the current one and switch to one that will give it to you.
And then they discover that transitioning undocumented business logic, retraining their user base, and adapting their business processes to use the new tools takes way longer than a few months, and indeed has a fairly high chance of entirely failing to dislodge the existing system.
Likewise, if you have half a million lines of code in a language, life will rarely be as simple as discovering that you need a feature and then switching to a language that has that feature.
For new project starts, switching languages is fairly easy. For individual developers, it may even make sense to switch employers to move to better tools. But if your business runs on a particular codebase and that codebase has grown large and powerful over a significant period of time, you throw it out and do a rewrite either after very careful consideration, or at your own peril.
Or they'll hire someone else who is willing cut even more corners to hack the thing together in a few months, but leaves the codebase in an even worse state.
That you're trying to screw them. They will then proceed to hire someone else who will happily do in a few months what you claimed would take years. It'll probably involve throwing out all your code and starting from scratch in the new programmers favorite language/framework du jour.
To extend concept to programming languages: When you need / really want a feature and the language is failing to give it to you there's a decent chance you'll throw out the current one and switch to one that will give it to you.