The killer feature I haven't seen with many of these solutions is easy, out of the box integration with internal systems (Atlassian Confluence, JIRA, Remedy, SharePoint, FileSystem, Intranet). When you have a SaaS search engine it's difficult to export that data... Even worse to secure it. Ironically, Plumtree Software (bought by BEA -> Oracle) had all of this in their product in 2001. What's old is new again... Those features are prime for a comeback.
I think this is a space where Elastic can do well with an on-prem or managed cloud offering that is "behind the firewall", integrated with customer's environment. Add in term vector search support, ML for document/query understanding, and integration with customer's security model (Active Directory) and it would be compelling.