Wall Street has always been home to some of Sun's and IBM's largest corporate accounts. I don't doubt Linux and/or BSD can do the job that Solaris can in some cases (with caveats), but it will take years
for that to happen. A "Linux stronghold" is misleading at best, TFA doesn't even support the claim.
And Linux will never replace mainframes. Nothing will.
At the risk of being modded troll, OO Calc will probably never replace Excel - other than Suns and big iron, corporate america runs on Microsoft Excel (not necessarily a good thing, but still).
OTOH, I know companies that are still running their websites and outward-facing interface systems on hardware and software that could be easily replaced by off-the shelf open source stuff, which will probably save them a lot of money.