You are right, this is not going to be trivial and you definitely need buy-in on a management level. Backstage is pretty malleable but the only way to get it to work in an organisation if there is only a single source of truth for things like your software catalog.

If there is a lot of legacy in your org, or if there is not enough buy-in then it will definitely be "yet another" system. There's a lot of functionality to unlock once you have a bunch of data in Backstage, though, so that could help tip the scale. The cloud costs dashboard that helps to show how much every service costs is a good demo :)

One benefit to Backstage's approach is that you don't necessarily need to change your processes and tooling to work with Backstage. It can help, but it's not a strict requirement. Other tooling in the space requires you to buy in to their specific workflows and so on whereas Backstage is a bit more agnostic.