Excalidraw covers the part of Miro most people actually use: a shared canvas for diagrams and rough thinking, with a hand drawn look that keeps a sketch from reading as a decision. It is deliberately much smaller than Miro.
What you are replacing
Miro sells the infinite canvas plus everything built on it: templates, voting, timers, facilitation for a workshop with twenty people. If you run workshops, that scaffolding is the product.
For architecture sketches, flows and the diagram that has to go in a document tomorrow, it is faster than Miro and produces something that looks like a sketch, which is honest about how settled the thinking is. It runs in a browser and can be self hosted, so a diagram does not have to live in a vendor's account.
What it does not replace
It is a drawing surface, not a facilitation product. Templates, voting and structured workshop tooling are absent, and a large simultaneous session is not what it is built for.
Stay on Miro when the canvas is where meetings happen rather than where diagrams get made. Replacing a workshop tool with a sketchpad is a downgrade dressed as a saving.