One point already made is if the ELFI is unusually high it typically means something is wrong. Something that ought to be fixed and not left to the added complexity of the RCD to protect folks against.
More generally though for lower current final circuits if you can't meet the Zs for disconnection then often you are going to be failing on VD as well. If you assume R2 and R1 are roughly the same, and much larger than the supply ELFI, then if you meet a 5% VD then your fault current is going to be around 20 times the working current, enough to (just) meet disconnection on even a D-curve MCB.
Of course if your R2 is higher, say T&E with the reduced size CPC, then that is no longer true but for typical domestic circuits on B-curve MCBs it will be easily met. In fact the On-Site Guide books Table 7.1(ii) makes it simple to check.
There are good arguments for high current final circuits or sub-main where you struggle to meet the ELFI due to the supply Ze value, even with easier OCPD such as BS88 fuses, where an RCD is quite a reasonable design solution.