For a variety of reasons, not the least of which is the desire to reduce the memory footprint by shedding unneeded features in some applications, it would be better if ESP would release the source for this library and/or document the low-level interfaces required to implement an alternative.
This would benefit the community obviously.
However, it would also benefit Espressif as they would then be able to tap into a multitude of developers that would contribute improvements and additional capabilities to the library over time and it would simplify the process of keeping the ESP8266 version of LWIP library up to date as well.