Loki and grafana? (not really an alternative in the sense of "full text search" over logs, but an alternative in the sense "an interface for aggregate search over logs").
There's also Cloki, which offers the same Loki APIs but stores data in ClickHouse. It looks interesting.
It does look interesting, but maybe a little too exciting: "Super experimental". One advantage of loki, is that it is part of hosted grafana, so there's some commitment to it as a product.
Grafana started out as a fork of Kibana 3 if I remember correctly, which was around the time Elastic acquired Kibana. Since then, it has become a nice product in its own right. But the shared history is interesting.
Elasticsearch, Kibana, and Logstash all have OSS distributions, and are included as part of Open Distro.
Fluentd (https://www.fluentd.org) is a great alternative to Logstash that I've had a lot of success with. It is easy to pair with Elasticsearch and Kibana and myriad other inputs and outputs.
I've personally found Fluentd to be flaky at scale or under heavy load, but it does talk to damn near everything and is pretty simple to stand up