And if not 10, then 11 or such. But 10 feels like the perfect spot.
The bonus? Any troops from the starred kingdom that you have at mythic and 4+ no longer drop from chests.
This allows players nearing completionist to reduce the size of possible troops from a pull, and thus better focus on completing the last few.
10 stars sounds like a reasonable point. In order to be able to get a kingdom there, it needs to have available something like 15 troops? And you’d need to be pretty far with levelling and traiting them. Before those kingdoms become possible, the current huge stashes of players will have been normalized, meaning it is an actual achievement to get there for one kingdom, let alone a multiple of them.