This is personal speculation, but the vested interest in continuing to market a lot in Japan may be to establish the brand for merchandising reasons, not just video games sales. Plenty of Persona merch and other things..like even a cafe?
Pokemon and Final Fantasy spend
a lot on global marketing to get the sales that they do. That would take a a lot of investment and risk from Sega to do this with Persona 5. To stay on topic for this thread still....judging by Sega's annual report, doing something that expensive and risky does not bode well with their current business plan and vision. They are trying to emerge from a downturn and likely going to be cutting costs. :/
Remember how Persona 3 and Persona 4 released on the best selling console of all time? That beautiful <1% attachment rate!!
I agree with this. Judging from past performance, I do not see Persona 5 exploding as some people believe it will. But that could definitely change if Atlus goes above and beyond with marketing and distribution. It could also very well hit super-cult on its own.