Do not give them ideas, on the contrary, give'em more rope. And when the time comes and they come with the idea themselves present yourself as the unique person

Do not give them ideas, on the contrary, give'em more rope. And when the time comes and they come with the idea themselves present yourself as the unique person that can do the migration to Excel, since meanwhile the codebase would be in millions of lines of code and you're the only one left to understand it. And that's your moment to ask for the big bucks as a contractor. Always remember the golden rule of life: stupidity and intelligence must be paid (their stupidity and hopefully your intelligence).