"How will Final Fantasy XV affect the FF franchise, from a critical standpoint, fanbase standpoint and financial standpoint?"
Franchise: XV is the first action-oriented in the mainline of the series, so as others have said, XV's success may convince SE that going in this gameplay direction is OK. This is not to say that all mainline games will be action-oriented from now on but that this kind of system is acceptable from time to time, depending on the director. (Also, VIIR is already headed toward a more action-based system.)
Critical: XV has two legacies to live up to: the FF and Versus ones. The latter is debatable since most of us really don't know how Versus would have turned out, but to ignore the fact that some people still talk about Versus (and others pining for it, right?) and XV in the same conversation is unwise. Reviewers may grade XV more meticulously, if not more skeptically, than previous entries, simply because: 1) as APZonerunner has said, there are now more games in the market that rival FF and are thus the basis for comparisons, and 2) it's been almost 7 years since the previous offline FF game has been released, and that's a big gap to fill. Whether we like it or not, and regardless of the fact that XV really isn't a decade-old game, people will ask the question we all have in the back of our minds: "Was the 10-year wait worth it?"
Fanbase: Most will like it, others will hate it with a burning passion, and some other "fans" will troll about it without having played it. Same old story. What matters most is that XV draws in a new generation of fans. Unfortunately, it's now not enough just to cater to old-time fans who've grown up and are now working 9-5 jobs. The action-based system and world-class production values are good starting points for that.
Financial: Without a doubt, the console market is shrinking in Japan, so SE have to set eyes on the entire world, not just their home court. XV's worldwide launch should help with the financial aspect.
Don't ask me where I read this since I can't recall, but I read somewhere that Tabata said that he and his team have been streamlining a lot of things in XV's development and have been trying different things for the sake of future projects (paraphrase). As far as Tabata is concerned, XV is a game about the future, with some traditional FF elements for the old nostalgic effects. It's not a throwback game to help fans relive the golden days of FF and whatnot. With all that said, XV is very much an experiment for SE. Action-based game, project development cycle, brand-new in-house game engine, etc. In some sense, the game is an anomaly and is likely not indicative of future FF games.
tl;dr XV may not make or break the series, but as far as SE's concerned, it is a critical stepping stone for the future of FF. If it gets good reviews, draws in a new generation of FF fans, and helps SE make bank, then it will have done its job.