Sorry, but I do not really buy the "SQEX wants to focus on the West when it comes to DQXI" story. SQEX tried to impose the IP on PS1, PS2 and DS and the conclusion was that, even on hugely successful hardware (one of those being the home of best-selling jRPGs), the IP kept selling way lower in the West with respect to Japan, with a maximum of about 1 million units.
Now, assuming that SQEX does not want to lose many sales from Japan, that would mean keeping the mainline entry around 3.5-4 million units. Which hardware will be able to support such sales in 2016? One might imagine that PS4 will have at least 6-7 million units around by 2016, which is quite unlikely given its current trend, and even if it starts selling well (around 30-35.000 units per week).
Realistically, how many units can be extracted from Western countries? Even though PS4 is hugely successful here, it still has to prove is the right platform for jRPGs; PS3 and 360 did not have any really successful jRPG outside Final Fantasy XIII, and many successes (if we want to call them like that) came quite late in the lifecycle, e.g. Ni no Kuni, Tales of Xillia. Lost Odyssey sold well but was really targeted to the West, unlike Blue Dragon, which indeed sold way less.
If DQVIII and IX sold around 1 million units in the West it is unrealistic DQXI will sell much more on PS4. So, by going exclusively on PS4, SQEX might face two problems: selling less than before in Japan (we saw how DQ typically benefits from huge installed bases), and selling less-than-expected in the West, such that it is not possible to offset the lost in Japanese sales.
We must also consider how SQEX has always considered DQ an IP with huge margin behind. DQVIII has probably been the only technically involving and expensive entry so far; developing a game exclusively for PS4 requires a lot of work in terms of time and money, in particular if the team wants to guarantee the typical big world-map.
So my bet is that DQXI will be on both PS3 and PS4, in order to exploit the former in Japan and the latter abroad. The risk is here is that the choice of PS3 might be out of maximum time; but I guess that people will not have problem to buy such a huge IP on PS3 in 2016 anyway (like they bought MH3P on PSP in late 2010 / early 2011).