Basic Rule was developed based on a lot of experience with and hopefully some insight into how new users can most easily approach an inherently difficult topic: creating automations. My wife, who is completely non-technical, can use Basic Rule and understand the concept of When Something Happens Do Something. The gap between that more intuitive and simpler UI and Rule Machine might as well be the Grand Canyon. The user audience for these two apps are completely different in terms of their knowledge level and experience.
The OP raises a valid suggestion. OK, I figured out how to ride the bike with training wheels, now I want to go further. Why should I have to start over? This is a natural point to reach as one learns their way around home automation. One can't push Basic Rules into more complex automations without losing its ease of use. Trying to add a simpler and more intuitive UI to Rule Machine is incredibly difficult. Creating a bridge from one to the other is probably a doable project and one worth doing. It would also be a great training tool, to see how a simple automation in Basic Rule can grow up into a Rule Machine rule.
Meanwhile, back at the Rule Machine Ranch, efforts will continue to improve its UI and make it easier to use. But let's not kid ourselves that it can somehow become a beginner's tool with just a little more polish.