Adding a second mode or an "option" to the game would mean needing to make two completely separate builds for testing balance and mechanics, and to support both modes. That is essentially doubling the workload of the design and QA teams, and doubling the time and resources needed, and likely reducing the quality of both.
That is more work than essentially reverting a lot of their changes closer to 5e and then making smaller tweaks from there to keep it closer to 5e, but still fun for most people.
