To me a huge goal here is to make stuff more extensible, ultimately I do not think the “simple” design should replace what we have now. I do think we need to cull out stars and quieten down categories in our default view, but ultimately there are different types of consumers for Discourse with different needs.
As you simplify, you lose information, which may be desirable to some communities and not desirable for others.
If a community say moved from years on phpbb or what not to Discourse it may make sense for them to use a “simple” design. If a company has a need for an aux column it may make sense to use a “simple” design. Long term I hope the “wizard” you use to setup Discourse allows you to select between a few themes that cater to different groups.