Agreed. Running Java does not excite me either, as there have been vulnerabilities in Java over time on various platforms.
I had asked to have help (or to fund) a true Sphinx add-on, but nobody has stepped up. We have the server resources to run Java and ElasticSearch with no issues, but compared to Sphinx, it uses far more resources (CPU cycles and memory). I just need to make the most of our hardware at all times, and the ES/Java combination is highly wasteful compared to Sphinx.
Not to say ElasticSearch itself doesn't have some nice features--it is highly configurable in ways that Sphinx is not. Definitely a trade-off.
I get that impression also. He did a LOT of custom work on his site, and if he'd used Sphinx rather than ES, he'd likely have offered up an add-on by now. I have to admit we're trying to get some funds together to be able to download some of those add-ons he is offering--many would be quite useful to us!
When I viewed "top" on the server, Sphinx was often in the list of 40 or so entries (that I could see--the rest scroll off the bottom), but never near the top. Memory usage was much more efficient since it was primarily disk-based, and still quite fast. I know the overhead of Java/ES is much higher, but our server can handle it for now.
Interesting though, while looking at "top". Even during peak hours yesterday, I noticed as usual that we might have two or three dozen httpd processes near the top (sorted by %CPU, which is the default). But mysqld always used to be at or near the top in the list, often using 50% or more of the CPU. With XF, now that things are tuned nicely and we run the database from a separate SSD, I don't even see mysqld when sorted by %CPU! (It currently uses 36% of available memory.) With vB, it was usually right on top. Now I just need to tweak a few more minor things and we should be doing quite well. We still don't exceed 4.00 server load (quad core CPU) even during peak hours.