It's a question of path dependency. A least-cost grid with 80% less GHG emissions than current will likely be, depending on the location, lots of wind and solar, improved transmission, a little bit of batteries and demand response, and then NG backup.

For a deep decarbonized grid (95%+ less emissions), the least-cost will OTOH likely involve a rather high fraction of nuclear, plus a somewhat smaller fraction of the aforementioned wind, solar, transmission, demand response, batteries on top.

Dealing with climate change requires a deep decarbonized grid. So while most grids can take a lot more wind and solar than currently, if we overbuild those sources we can get locked into higher emissions long-term.