Honest question: why are you expecting PS4 to be a huge jRPG machine in the West such that DQ can finally find mainstream success, something that didn't happen even on PS2 and DS? To be fair, PS4 Western userbase seems even more skewed towards Western IPs wrt to PS3, which built a niche jRPG fanbase over time, and not in the first few years.
I'm not necessarily expecting. THEY are expecting.
Honestly I think that up to now there was almost no data to judge if this idea is good or bad. I think that the SE support still has to come, so we don't know if the strategy will pay off or not.
But the strategy itself is pretty evident imho.
I'm pretty sure that a PS4 exclusive DQXI will sell a lot less in Japan than a 3DS exclusive DQXI. If you put PS3/Vita version too in the equation, I suppose that the gap would still be there, but smaller. If you count Western sales on, I'd say that a PS4 version (don't count on PS3 version to be localize, if DQH is an indication) and a Vita DD version (even here I'm baseing this on previous examples) could somehow be on par with a 3DS version, considering how "big and active" the PS4 userbase will be in late '16/ mid '17 compared to the 3DS one (not saying that the 3DS isn't viable in the West, but it's evident how the PS4 has an even better pace at sales in US and EU, and I think that for when DQXI will hit the market, the gap in terms of "active" usebase will bre pretty significant)
I'd also add that probably as Nintendo was able to convince (I'm not talking about money-hatting here) some big Japanese publisher in putting their "Japanese-centric" games on DS/Wii/3DS thanks to a localization/distribution/publishing Western support, probably the reason why there will be no 3DS version of DQXI is pretty similar (to backup the obvious smaller sales in Japan)
EDIT: to be more clear: I think that the 3DS will not anymore be a HUGE JRPG environment in the West for when DQXI will come out, and probably PS4 environment in the West will be healthier/more active/bigger, especially considering all the JRPG coming in the next years (from S-E). If this will backup the smaller (I'm pretty sure about this) Japanese sales, I don't know