I think the most resistance would be from marketing/sales and customer teams who have already become well used to the system and are in a comfort zone. Not to mention the development teams worries about their role.
Next step becomes easier if we demonstrate a viable alternative modern system which allows easy phase by phase replacement of legacy system. A pilot project with willing internal/external customer can help top management and other stakeholder see the advantages.
But when replacing system entirely one should never discount the possibility redesign/re-eengineering some components can help us avoid complete overhaul when really not required.
Also important is to consider how much investments have gone in to the system and what is legacy. Do call a 2 year old system or 5 yr old or 10 yr old system as legacy???? I think that's a call which can be specific to the legacy under consideration.
Hope this helps,
(Excerpt from my suggestion on IASA-Linkedin Discussion