Money is one reason Europe is a guaranteed success, Japan is also closer to NZ and Aus than Europe they're all long haul flights but 2019 isn't a world cup for any of the tier 1 nations. Having it in Europe guarantees an integrated World Cup considering how easily travelled it is and the SA, Aus and NZ populations only compliment that. I'd argue that with rugby's current stature the only 'World' Cups we have from the perspective of attending fans are the European one's. Obviously that's a simple matter of geography and nothing else.
There's also the question of who 'they' are, as far as I'm aware the host of the RWC is decided by a vote between the Unions. Ireland would certainly have the advantage there, getting votes from the home unions and I'm sure that the match in Chicago later this year influences the NZRFU vote too! Rightly or wrongly, probably wrongly, I think the 2023 cup will be here, I hope so too as I think it'd be Ireland's first, last and only chance to host one of these things before the stature of the game outgrows our infrastructure.
After that we'll probably not see one in Europe for another 12 years, 2027, 2031 and 2035 will surely be shared between Argentina, North America and Australia. Considering at the last one of those I'll be twice my current age that's a conversation for another day though!!
Edit: Saulan, if Ireland were to get it it wouldn't really be in the UK, a few group stage games would be in Belfast but the business end of the tournament would be solely in Dublin. A Dublin world cup would be different but it'd have a bit of an Olympic vibe to it and certainly be a better place for it than London which was too big to achieve the rugby loving atmosphere like Paris had in 2007.