I think this is a good idea, but perhaps the Monzo data guys would be able to figure out what threshold that yearly allowance would need to be at to break even.
If most people only travel abroad 1 or 2 times a year, then this would benefit them. Anyone who travels on a far more regular basis would then need to start paying for these transactions.
I think this logic works if Monzo continues to be a bank and not an international travel card, as it has set out to do.