I see Penpot primarily taking Figma's market share around the edges. For established design teams with robust workflow needs, there's no competition right now despite some of Penpot's novel design features like grid layout. For independents, enthusiasts, and teams running on a shoestring, however (read: Figma's free-tier customers), it becomes considerably more compelling as a potential alternative.
To paraphrase Reed Hastings' classic line: Penpot needs to become Figma before Figma can become Penpot.