I can speak best about the technical side of things (in which I am most involved, together with Luc Döbereiner), I can say that sustainability / long term support has always had a very high weight in the decisions we make there. It has, for example, been more important than adding new features or supporting the very latest web technologies.
Recently, there has also been a focus on making the content/expositions itself exportable (in which an export should still be viewable without the RC website, if such a day would ever occur). This has currently resulted in a HTML export, that one can use to export an exposition in a way so that you could host it oneself, or even just view it offline. We are also in a dialog with archivists, to find the best way to archive our content for the very very long term (with this I mean more than 30 years, which for web technologies is an eternity).
From an organizational point of view: the Research Catalogue platform is a shared platform between a number of institutions spread over Europe and does not rely on a single source of funding. There are at least 17 portals on the RC now, and they all share the costs for maintaining the RC. There is twice a year "portal partner" meetings in which the portals discuss the current use and further development of the RC. At this moment the number of users and portals is growing (more importantly: the activity an publications in the portals themselves are growing).
So my personal opinion is that the expectancy for long term stability of the RC is good and actually one of the more attractive aspects of the RC as a platform!