I think your greatly underestimating what Persona has become. There was poll that Kotaku did on simply "Persona 5 or Final Fantasy XV" . The final results were..
19k Votes Persona 5
14k Votes Final Fantasy XV
So a pretty resounding show of the level of interest in Persona, in what is not a small sample size. Now obviously Kotaku readers who would be likely to vote in a poll like that are among the more hardcore gamers. I think Final Fantasy would do much better in a poll of more causal gamers at the very least due to greater name recognition.
But I don't think I'm going out on a limb saying that if Final Fantasy XV drops the ball the way Final Fantasy XIII and it's sequels did Persona really might take the throne as the new king of JRPG series.
There is also some irony in the fact that the guy hosting the Final Fantasy XV date reveal event is Greg Miller. Who has always been very open and transparent about the fact that he's a huge Persona fan and has never been able to get into anything Final Fantasy at all. Which isn't an accident. Square Enix really wants to reach out to those kind of fans. I think that's smart because they know they need to.
That is a very small number of people, considering the regions persona was released into. Even then, Final Fantasy is almost even. Persona, outside Japan, is still a niche title. You are grossly over-estimating its popularity from such a small sample size for FF audience. For Persona? Yeah, it is decent. But then again, that series hasn't been breaking records worldwide. About 50 % of the title's sales are from Japan. When it was released in NA and Europe as well. I am not saying it's not getting popular. But it's a long way off from FF. As I said, about 7 million sales (6.9 million) were attained by FFXIII alone. If I count all three XIII games, then it has sold over 10 million copies, and that from just three titles:
http://gematsu.com/2015/05/persona-series-sales-top-six-million
http://personacentral.com/the-persona-series-has-sold-over-6-million-copies-worldwide/
That is all 6 games sales figures combined, where more than 50 percent are in Japan.
Let's compare that to Final Fantasy:
http://vgsales.wikia.com/wiki/Best_selling_Square-Enix_games
http://vgsales.wikia.com/wiki/Final_Fantasy
All FF games have sold over 110 million copies; 6 million sales are literally worthless by contrast. Last mainstream FF title (XIII) beats out ALL sales figures of ALL Persona games combined. There is no way in hell Persona would come even close to XV in sales, when it's the most anticipated title in Japan for Famitsu, and if you have followed it on yearly basis like most FF XV fans have, you would know that it has almost always been the top most anticipated title by the Japanese public:
http://kotaku.com/the-most-anticipated-games-in-japan-1761450315
The hardcore gamer lobby is always very small. I hardly doubt it is that big as you are assuming. Nor are those numbers indicative of hardcore gamers as a whole. There is no proof of it at all. FF XV has been shaping up very nicely. So, I am not surprised that people are being swayed. Rumor has it that Persona is also being delayed, and rightly so. I highly doubt Atlus wants to complete with FF in Japan or elsewhere. It'll be a financial suicide.
6.9 Million sales and over 10 million as a whole is ... dropping the ball? Come on, man! Personally, I never liked XIII and the sequels. But, I am dead sure that I am the minority here. About 10 million people played and enjoyed FF. Lightning has been constantly voted as the favourite female character poll in Japan. As I said, this "dropping the ball" is a minority opinion, not shared by an overwhelming majority.
Anyhow, I am really excited for the March event. I need to see Magic already.