I do think each of the mane six, and Spike, should continue to grow and develop into something greater. At the same time, 'Princess' has a somewhat-defined meaning within Equestria and it's not clear that that path is appropriate for all of them. Princess within Equestria isn't the natural progression of all ponies so long as they grow and learn enough. It's not some title to award to all great citizens. Yes, it will provide structural balance from an overall narrative standpoint, but would that feel organic, what the characters-as-we-know-them would really want? They are an ensemble of friends, each with their own strengths and weaknesses. Their most valuable strength—and the show's core spirit—is working as a team
supporting each other. None of the mane six could be easily swapped around like some superhero cast, where any member could equally accomplish the task, just in a different flavor of lightshow. Being a part of a greater whole, without sacrificing individuality, is such a strong feature of this pony cast. Applying the princess stamp to all of them would be a step in the other direction.
As examples of where they might be headed if not princesshood, Rarity could continue her conquest of Equestrian fashion and society and become a de facto leader there, but that kind of leadership isn't the task force adventure-kind that Twilight usually does. Similarly, Rainbow Dash is already developing her leadership whenever she coaches athletic teams, weather teams, or little filly teams. Fluttershy is already quite indispensable to Equestria as an interspecies diplomat, bogging her down with official bureaucracy doesn't feel fitting; most of her growth would probably be internal. Pinkie Pie is a wild card..she's already key to the functioning of society, making her a capital-P princess would probably
diminish her ability to spread laughter and joy.

Applejack has leadership skills, but her character progression makes it less obvious where she's heading. There are many possibilities, but I haven't really noticed the show hinting that she's on any grand path other than 'make farm and family run smoothly'. Spike—I don't think anyone is expecting him to become a princess, but regardless, he could use more character development of any kind.
I have no doubt that making them all princesses is always in the back of some creative exec's mind because it's such an easy ending to jump to. It brings fantastic closure. And marketing and toy division would love it. However, any deeper look at the show's origins, and even as it currently is, would indicate that it'd be artistically outrageous. Pull-off-able, yes, which means it
could happen, ie. they
could force in yet another change in the definition of what a princess or alicorn means within the universe, but a positive change for the show's narrative integrity? I have my doubts.