Any explanation as to why this is unreasonable? In general it requires you to book the lowest reasonable alternative flight, but if that (on short notice) is Emirates - then the difference between paid amount for the original ticket and the replacement should be reimbursed. Also if this is the outbound, you might need to check if booking a full new roundtrip ticket would be cheaper, and if so, book that, but in general just because of the high disparity between amount paid and costs effected that does not make them unreasonable per se.

Greetings - Dirk

p.s.: I was in a similar situation with them very recently where I paid ~650€ for an EW flight and needed to rebook on another service due to their fault - they reimbursed me over 2.5K in additional airfare. Took about a week after submitting my documentation.