A failed title in a spin off genre wouldn't hurt the over all brand, look at Tiberium 3rd person shooter that got canned, and hasn't heavily damaged the C&C line of RTS games. A failed title in a genre you're looking to establish your company's future (and thus financial investments) in as more than a 1 trick pony is a killer. If FFXI dies tomorrow it dies a success, FFXIV can't say that, and if FFXIV dies a failure you can bet SE's future in the MMORPG genre is done. If SE's future in MMORPGs is done then the potential for huge profits is also gone.
Console games make profits but not to the potential of a successful MMO does.
Super Mario Bros is supposedly the highest selling console game of all time with 40 million copies sold. Lets attach the current $60 price to that number (yes I know this game didn't/doesn't sell for that much). So at the best of the best possible for a console game you've made 2.4 billion dollars over 26 years.
Now lets look at the best of the best profit wise MMO WoW. Last subscription base I've heard thrown around was 8 million users so I'll use that. At $15 a month that's 120 million a month or 1.4 billion a year, so in 2 years WoW makes 480 million more than the best selling console game could have made in 26 years.
Hopefully you can see why an investor sees the cost of fixing FFXIV as worth it and why they haven't just written it off.