To be fair, there is often a better reasons to only replace a portion of ES' functionality, since doing so can save a lot of computation and space; than to replace ES itself, since it already exists and does a good job if what you need is the full kit.

I found myself last week reimplementing 10% of RoaringBitmap's functionality as a homebrew replacement, because doing so was 500% faster. Not that RB isn't great, but it's designed for a general problem space, and not my particular problem.