Even if it offers only a fraction of the features offered by ES, that may be fair enough for at least half of the use-cases out there.
Sonic could have really had a strong selling point: "Use an ES-alternative that works fine in most of the real-world applications, but it's written in Rust and it only takes a fraction of the memory footprint required by ES, and it shouldn't require you to change your application code".
Instead, they are proposing yet another search protocol, that developers have to learn and adopt. That definitely increases the adoption barriers.