I think there can be a philosophical novel in this day and age, and it’s more important than ever that one is published. But it faces challenges, which Erdal says are essentially “balance” and the question of what it would look like to consumers.
On the balance of it, SF&F seem to be the most likely place to find the philosophical novels of the age. So much of what they have been preoccupied with for so long has been philosophical discussion of things that affects all of us: artificial intelligence, extraterrestrial sentient life, the impacts of technical progress, etc.
That said, very little SF&F is reminiscent of what I would call the "big novel of ideas" approach, the sort of thing the author of the above piece attributes to folks like "Tolstoy, Dostoevsky, Sartre, Camus and countless others". I will most likely surprise no one when I say I love such books. I'm in the middle of re-reading The Brothers Karamazov now in one of its new translations, and I see firsthand (doubly so in this new translation) what a novel of ideas can do at its most unchained and vivacious.
The paradox is that such books work because they are not novels of ideas. They are novels, period, with robustly depicted characters involved in things that draw our attention, and those things just happen to be big ideas. Such writing seems to have fallen out of vogue, if only because to evoke a "serious" idea in fiction seems to require that you treat it with manifest unseriousness. Can you think of a serious novel in English about religion since Graham Greene pulled the last pages out of his typewriter?
SF&F books that hew in that direction seem to end up in territory more akin to the "big family/historical saga" vein of story — very James A. Michener (or maybe Alexandre Dumas) with a dusting of Jack London for good measure. They rarely fail to sprawl or pack in the details, but even when they have a few key characters front and center they're mostly placeholders for the action and the ideas. And when it comes to the ideas, they seem to leave the philosophical side of their stories as a given. In other words, they expect the mere presence of issues to count as an analysis or discussion of those issues.
This is a common trap to fall into, and I think it's one of the reasons why a work can have the veneer of something philosophical without being that way itself. A work that contains a great many things without actually being about any of them will simply be an overstuffed suitcase, with too many of the wrong things jammed into it sidelong. This explains the proliferation of fiction about "issues" as opposed to ideas or, well, philosophies — the sorts of things James Wood found fault with in Zadie Smith, that B.R. Meyers shook his head over when he found it in Jonathan Franzen: "The critics do their bit by acting as though name-checks constituted themes and issues. I can hear the prize laudation for [Franzen's] Freedom now: 'It is a novel about commercialism, about the war in Iraq, about the pervasiveness of Hollywood culture …'"
Funny how Dostoevsky's book, despite being written a century-plus ago, is more "relevant" to me than anything in Freedom. Franzen's friend David Foster Wallace made a stab at trying to ask why Dostoevsky still rings so true in his piece "Feodor's Guide" a ways back, and the most he could come up with was some talk of "a passion, conviction, and engagement with deep moral issues that we, here, today, cannot or do not allow ourselves". That didn't ring true for me when I read it: writers today seem to be going bananas over trying to tackle deep moral issues, but they largely fail because they think of them as just that: issues. Any politician makes key "issues" a plank in his platform and can often do so without showing a sign he cares one real whit about the subjects in question; it's clear to me now novelists are no less capable of that kind of duplicity. (And then there's stuff like Atlas Shrugged, which is to philosophy as Naugahyde is to leather --and I'd argue a similar analogy applies to its status as literature.)
What really kills the project is how the formats created for modern "serious" fiction are so un-serious, how those books end up so loaded with coy wordplay and farcical contempt for the very people they claim to be so interested in. When I read Dostoevsky I feel he is serious not simply because he is tackling Big Themes, because it's clear anyone can claim to do that. His distinction is in how he gives everyone in the story a fair shake, gives them freedom of speech and movement, and allows the seriousness of the story to arise naturally out of those things. He was not mining the whole thing for coy little digs and cute little cheap shots. Today's authors are unable to think of a pet issue of theirs in any terms other than how they can most snidely caricature an embodiment of the other side of it — or, worse, caricature both sides. (Don DeLillo's oft-praised White Noise seemed like the nadir of this sort of thing.) They lack the empathy and the curiosity needed to provide a story about "issues" with the spaciousness and freedom of movement it needs. They take the cheap shots because they've been led to believe anything more ambitious than that is of historical interest only, that taking cheap shots is all that's left. "We shall not have succeeded in demolishing everything unless we demolish the ruins as well," said Alfred Jarry, and it seems his demolition squad is still drilling to China.
SF&F has the courage to tackle the really big, normative issues that get short shrift in other venues. That makes it all the more suited to the philosophical novel, but that also makes it all the tougher for it to live up to such promises. because of all the baggage SF&F carry along, some justified and some not. A big part of it is expectations and audience: if I write a book that depicts the next thousand years of human civilization after the invention of a drug that confers immortality, does that mean the book is about the drug? Or (let's go with a real-life example) a book about the thousands of years that pass after the invention of a science of the behavior of human populations, right when galactic civilization is about to take a nosedive? "Genre" readers wouldn't have any problem with a book that was just about the drug or the science; in fact, they'd be wildly curious about it. But literati would see that as a shortcoming; they'd demand more than just a story about the consequences of a given technological innovation. They are not wholly wrong for wanting more, but their criticism may go unheard for all the wrong reasons.