The TSDB it uses is actually pretty state of the art. I think your pain point is more that it's designed for being used on local disk, but that doesn't mean it isn't possible to store the TSDB remotely. In fact, this is exactly how Thanos works.
The docs say Prometheus is not intended for long term storage because without a remote_write configuration, all data is persisted locally, and thus you will eventually hit limits on the amount that can be stored and queried locally. However, that is a limitation on how Prometheus is designed, not how the TSDB is designed, and which can be overcome by using a remote_write adapter.
The docs say Prometheus is not intended for long term storage because without a remote_write configuration, all data is persisted locally, and thus you will eventually hit limits on the amount that can be stored and queried locally. However, that is a limitation on how Prometheus is designed, not how the TSDB is designed, and which can be overcome by using a remote_write adapter.