DOA would lose its credibility about the most costumes in a fighting game if they were to release it on PC. Trust me. I got friends that have said the exact same thing.
I have friends who play eroge and this doesn't even make sense to me. Pushing that aside though...
I'm all for a PC release.
With the arcade release I'd imagine they had to do some engine work anyway for the game. Someone above (sorry scrolling is hard D

noted that a lot of the arcade machines are running Windows (or some variation of) on the boards. Seems pretty reasonable to think they'd try this should most of the leg work have been done already.
The convert job all comes down to the code base. How much needs changing for platform specific (generally all your read/write stuff). What extra stuff do we have to include for this platform, etc.
Inside tip: Most developers build the game on PC's built around the current generation hardware. Developer kits are expensive and used primarily for testing builds, which take a long time in some cases to compile. At those game conventions when they are show casing something you're usually just playing on a PC built to XBox/PS3 spec.
So the code base should be pretty decent... it all comes down to platform specific calls and a few other things. By no means am I saying this is simple but its complexity lies in what they've done already rather than what they'd have to add.