I think this is a great idea from a user’s perspective & ironically, this might make users spend their money using Monzo more because in order for the streaks to mean something, when you spend you have to make the transaction from your Monzo account.
This might be a good extension of the virtual saving pots. But even if Monzo decide not to build it, it’s exactly the sort of thing that a developer could build an app for, once Monzo’s API is ready, a bit like these apps that have been developed already 