I just don't see where they'll get the fans from. Trying to carve out a fan base is difficult enough. The Pro12 will be an even harder sell to fans when the option of attending the Premiership is nearby.
There's over a hundred thousand people in London born in Ireland. Count in Scots, Welsh, and people who are English-born but define themselves as one of those heritages and you're maybe looking at a quarter of a million?
That's the target audience. People who'd genuinely prefer to watch a Pro 12 game over a Premiership game most weeks anyway. Throw in casual fans around SW London who fancy a game when Quins are away and there's something doing there.