Regarding the adoption rate of S, I think you hit upon a key source of the complaints about R.
R existed in one form or another for a (relatively) small, specialized audience for a long, long time. This included the period where transitioning from S to R included placing a very high priority on maintaining code from S, even being able to replicate the bugs from S. This made a lot of sense from the perspective of a (relatively) small group of statisticians making a data analysis language for their own uses.
Over the last 10 years, R has become massively more popular, and many, many more people are using it outside of its "base" audience. Not surprisingly, many of these new users are discovering that R was not designed with their personal use cases in mind.
Given these constraints, R has done a remarkably good job of adjusting (where it can) to accommodate the diversification in its user base. But you can only go so far without just re-writing it from scratch, and the interests of R Core will always bend towards themselves and their, well, "core" audience.