California generally has a good track record of hitting longer-term environmental goals. 2045 is a long way out, but it is a length that will allow for unclean plants to see through to their end-of-life, rather than kneecapping current plants/projects. I suspect the intent was to signal to energy producers that CA won't make their current investments worthless (otherwise it would drive away developers who have put investments in to our grid).

In other words, this was a "keep investing, but new plants need to be clean" move.