Most users won't actually tinker with the software, but some will. But that only happens if they have the freedom to do so. It's an essential property of long lived OSS projects. E.g. Linux, Mysql, Postgres, Python, etc. all have developer communities spanning many companies. Some are more dominant than others of course, and this changes over time. Companies going bankrupt is not fatal to these projects. Developers move between companies but keep on working on the software. E.g. Guido van Rossum worked for Google, Drop Box, and recently for Microsoft. None of these companies own Python or seek to control it but all of them benefit from it.

The thing that goes wrong in so-called OSS companies that attempt to monopolize the rights to their software (by e.g. insisting on copyright transfers for contributors) is that it leads to developer communities that are mostly just employees of that single company. Of course, most bigger commercial entities would probably not commit a lot of resources to delivering and transferring IP to another company; for free. This kind of requires a more robust setup with proper governance.

Such projects do exist, and most of them are backed by (very) successful companies who indeed don't get to monopolize the projects they are involved with. That kind of is the whole point.

The problem that the likes of Mongo and Elastic have is that they compete directly with such projects and need to add (proprietary) value every year to ensure that these projects don't catch up to the point where users can choose to simply switch to those and not really lose out. Elastic has plenty of OSS competition. There's of course Solr; which is based on the same OSS core (Lucene). Then there are several databases that integrate Lucene. The whole observable software movement has spawned several competing stacks (e.g. Grafana) for Kibana as well. Even Mongo has search features integrated (through Lucene). Likewise Mongo has to deal with other databases constantly adding features. Particularly postgres is very competitive and has added lots of features in the last ten years that were once key selling points for mongo.