Porting is not necessarily (speaking as a primarily Linux developer but writing portable code targetting several platforms including Mac and Win32) the major difficulty. It could well be .NET running as Mono on Linux on AWS, it could be C++ using a cross-platform toolkit such as Qt (the non-GUI components, not the widgets) - either would be eminently portable. The front end servers could just be Apache + PHP offering an API, which runs on Windows as well as Linux.
The rub would be, as Stephen points out, squashing at least two layers of server processes designed and tested to run on separate AWS instances into something running inside the game client process or as a standalone local server process which the client would connect to locally. There will be something offering the game API to the game clients as above, a separate (from the API for crash and hack insulation) process running the background simulations, and at least one backend database for player states, galaxy states, cartography, outfitting, NPCs and missions to live on. That will be a big database optimised for the huge load of hundreds of thousands of players in the online game and won't just be easily packaged and deployed on a player's local machine. Going on Frontier's job postings it's probably an enterprise NoSQL database which was never designed to run embedded. We tried to run one of these (OpenLink Virtuoso) as the local, per-user backend for KDE's Nepomuk semantic desktop index and it was never good and reliable, so after 5-6 years of development it was ripped and replaced with a less ambitious bespoke server. It's not just a matter of taking the game DB schema, banging it into SQLite, and telling the client to connect to localhost instead of the AWS server.
Bringing all this fabric together reliably on a player's machine would probably take FD another year, and then the people with weaker machines would find it was starving the game client of CPU and I/O bandwidth.
TL;DR: Since ~ Alpha 3 the writing was on the wall for offline-only mode; we just never looked at the wall. Best move on.