I'm sorry if this sounds snarky but a lot of your criticisms of slack so far just don't seem rooted in anything resembling the recency of development within the platform.
https://slackhq.com/threaded-messaging-comes-to-slack-417ffb...
Threaded conversations have been supported for a full year now, you have the ability to specify and narrow your searches down to specific rooms by using "in:#general" or by conversation by using "in:@dave", but a plain search will search as best the application can public rooms, private rooms and private messages, and the application does a great job I think telling you exactly where the search results come from, when they happened along with a contextual view of the conversation that happened before and after the search results-giving you the precise context of what was talked about, and allow you to both jump back in time to the moment the keyword appears, and jump back to "Now" very easily.
https://slackhq.com/next-level-searching-in-slack-1ce56aa8ad...
Again, sorry if this sounds snarky, but I don't think your critiques are really lined up with how Slack has been developed in the last year. Many of your problems have been directly solved.