Quote - So why not just get rid of included Renderers ...

For the same reason that they include figures and clothes with the program, anyone just starting out doesn't want to have to go and get bit & pieces from elsewhere, they expect it to be usable "out of the box". A second reason is that Lux doesn't do NPR (cartoon, sketch etc.) which a fair few people want.

Even if SM decided to go with a modular architecture, a renderer would always be part of the base (much like the figures etc.)