Because you don't "just" develop your next project in some other language/framework. You and your team have to throw out domain knowledge and make a whole bunch

Because you don't "just" develop your next project in some other language/framework. You and your team have to throw out domain knowledge and make a whole bunch of mistakes with the new language/framework before reaching the competency you "just" threw away. "Just" is a terrible word we use far too frequently in the software industry.