The Publishers of Ryzom were in a very similar situation, and their decision to open source the client and server proved to be what they needed to keep their game afloat. Aka "Ryzom Core".
Ryzom is even older than FE, and has been open source since 2010. It proved to have a small, but very energetic open source development community keeping the development pipelines up to date, keeping it able to build under modern versions of operating systems freeing the paid staff to develop content and focus on operations.
The funding for the switch to open-source was done via a successful crowd-funding campaign, where 172k Euros were raised for the transition. I figure there is a transition as open sourcing means taking spacial care with any licensed technologies being used in the existing client and server. I dunno the nature of the limits you have with the legacy Icarus code, but I am thinking if it is proprietary, and it is yours now, then the limits on open sourcing may be few.
At any rate, it saved Ryzom form the grave, and that game is still here, and they managed to modernize their code base and toolset as a result rather than port it to something new,
I mean porting to new engines is a LOT harder than anyone thinks. Many MMO projects have tried it, and cases of it succeeding are rare. (like how the attempt for The Repopulation was a train-wreck). The only case I can recall where it worked was Anarchy Online but it took like 6 years, and they had all the resources of Funcom who was a lot better off financially and could support the project.
Anyway, Ryzom/Ryzom Core is a real world example of open sourcing a MMO actually worked to save the game, and has kept it going since. Maybe worth seeing if a similar path could be followed?
