trello.com vs monday.com
Both are trivially easy to clone (the UI/UX is straightforward) but nearly impossible to dethrone because network effects and switching costs are brutal—so you're basically tied in a "build something better AND convince 10M users to abandon their muscle memory" death match.
Trello (Atlassian)
10clone difficulty
moat 103+ months with a team (for a basic kanban clone with real-time sync); years for full feature parity$100k+ for MVP kanban clone; effectively unbounded for full Trello parity
You can clone Trello's homepage in a weekend, but you'll spend the rest of your life trying to clone the drag-and-drop sync engine and Butler automation that actually make it a product.
Full teardown →vs
monday.com
10clone difficulty
moat 103+ years with a large team (for the full product); a weekend for the marketing page alone$50M+ (full product, historically); $500-2k (landing page clone only)
You can clone the billboard in an afternoon; the actual $1B+ ARR work-management engine behind it took a decade and hundreds of engineers to build.
Full teardown →