I am all for advanced memory management in application frameworks. The OS however isn't something you're going to rewrite very often so work harder and squeeze out efficiency so that your apps get them.
OSes are constantly in development. Making development faster and easier could very well lead to a more efficient system. Faster and easier development makes it easier to test out new ideas and spend more time on optimization since it took less time to make the actual feature. Sure, if hardware never changed and the feature set never changed then you could just write the whole thing in assembly and make it as efficient as possible but that isn't reality. When it comes to software requirements and limitations are always changing so making it easier to adapt to those may in the end be the more efficient solution.
why not? if it's all hot swappable and development at that level is as easy as in user space