Against Dash and Satoshi's design (e.g. Bitcoin) that can theoretically be executed with a much less costly Finney attack (where the attacker wins a block but doesn't announce it right away and first announces his double-spend, which is even more likely in Dash's InstantX because the confirmation is instant making it much more feasible to fool the unwary merchant who was assured that InstantX is instantly confirmed so not to wait for chain confirmations), so no need to invest such massive resources. And there are other less costly attacks specifically on Dash that monsterer alluded to and I will be following up on in future posts.