And yet it is in almost all respects more capable than Notion, especially now with Columns and Embeds (both recently added). What in your view makes Fibery less “fleshed-out enough to go primetime” vs. Notion and its 10s of millions of users? To my mind Fibery has already been trying to “go primetime”, it just hasn’t found the way to sell its actual capabilities to the right customers to gain significant traction. Nobody so far knows exactly why, but I do know Fibery has not tried significantly to take the Notion “appeal to solo users to get to business case” approach, and I think there is serious potential in it. Michael has expressed both interest in and skepticism of this approach, so far.
While I don’t agree entirely with your previous paragraph (about embedding Fibery, etc.), I do think “collaboration with the outside world” is involved here. In particular I personally think that making Fibery a non-exclusive aggregator and manipulator of other systems’ data could be very powerful for driving adoption. It does not force you to choose as is currently the case (as you pointed out). For this I think one of the biggest missing capabilities is (more) Bi-directional integrations/sync.
This is a good and interesting way to frame it, “hard convert” vs. “soft”. In general I agree that Fibery needs more “softness” in its conversion path. 
Don’t force people to choose too early. Make it connect easily with their existing tools and workflow, so people can use Fibery only for what they need/like at the start (which may be different for many). Then they use it consistently as part of their other workflows and as they do, if Fibery team has done its job right, they will see more and more how well-designed and capable it is, and they will also have a gentler onboarding and learning process.
If you do not need to build your entire company/workflow (or, critically, not even an entire department or area of work like product dev, issue tracking, etc.) into Fibery, but can instead literally use it for “just” visualizing Notion data (because Fibery has graphs and Notion does not!), or “just” adding whiteboards to Coda data (because Fibery has whiteboards and Coda does not!), or “just” any other smaller things, and (critically) if changes in Fibery can sync back to those tools, then it becomes a reason to not just use Fibery to view/analyze data, but to adjust it too. Soon maybe you find that adjusting data in Fibery and immediately seeing analysis update is much more pleasant than using e.g. Notion’s database anyways. And it grows and grows… There is currently no such flywheel, no such gentle onboarding flow, IMHO.