You cannot compare Persona to Yakuza in terms of sales, it usually takes 3 or more Yakuza games US sales to match one Persona title.
And SEGA have shown to be fine with sales of 100,000 or more, otherwise titles like 7th Dragon or Shining would not exist. I know everyone likes to believe that just because Yakuza 5 was not brought over that means SEGA are going to cut out every other title but Yakuza is not the same level of niche as Persona nor is Yakuza 5 like the previous entry (30 hour long story mode, 5 hours worth of playing as a girl trying to become an idol)
And just to hit home the size difference between Persona and Yakuza, the Persona released on Vita sold more in America than Yakuza 3, 4 and Dead Souls.
Maybe I can't, but a successful title to ATLUS may not be so successful to SEGA.
They seem to be happy not localising those games to the west either despite being happy with 100,000 or more in sales in Japan, and they're probably less of a risk than Yakuza if we look at it that way, especially 7th Dragon which looks like ATLUS' own Dungeon Crawling series. (I forgot the name of it. >_<)
I don't personally believe that Yakuza 5 is the be all and end all, but Nagioshi's own Studio hasn't made a game that has sold that well in the west. Not saying he will pull the plug on Persona, since it does sell much better, but I still have my doubts in his decision making if Yakuza, the franchise he wants to do well Worldwide, hasn't managed that due to many different factors in the west.
That makes me wonder why SEGA would release something like Yakuza in the west for consoles, knowing it sells low numbers but won't localise any PSP projects, which are potentially less of a risk. I understand the console itself isn't that big here (bigger market in Europe though), but surely it'd sell more than a single Yakuza game? Valkyria Chronicles II did roughly around 100,000?