Some ideas are terrifying enough to warrant a preemptive strike.
A reboot of the Poirot franchise would fail on several counts. Some would only be true if they chose to modernize him, a la Sherlock, some only if they tried to recreate the spirit of the original series.
But any one of the below is powerful enough to be fatal. To the rebooters. Because I would kill them.
Reason #1: Xenophobia isn't cool anymore
By Poirot's own admission, one of the principal factors of his success as an investigator is his image of an innocent, ignorant foreigner.
Poirot operates in a time when the Belgian refugees from the predations of the Hun were thought of as ineffectual man-children. They were curiosities from abroad, certainly deserving of our sympathy and respect, but surely incapable of Holmesian genius.
This attitude which Poirot must confront in almost every case is probably equal parts xenophobia and caste snobbishness (the latter of which is still in full bloom throughout Western culture).
But I still say that an essential element of the world of Poirot is his separation from his subjects by national identity. Nationalism has also fallen by the wayside in our time, a sometimes-welcome by-product of the shrinking of the world due to technology.
Without foreignness as an ever-ready misdirection, Poirot loses an important edge with his suspects. As he says in Three Act Tragedy:
It is true that I can speak the exact, the idiomatic English. But, my friend, to speak the broken English is an enormous asset. It leads people to despise you. They say – a foreigner – he can't even speak English properly. […] And so, you see, I put people off their guard.
Reason #2: People would see Poirot as derivitive of Monk instead of the other way around
Poirot exhibits many obsessive-compulsive traits, especially in the BBC series: having his books arranged by height, adjusting the placement of ornaments on the mantle at a crime scene, his fastidious appearance. But Christie wrote in a time when it was okay to laugh at such things, instead of tut-tutting at a mental disorder.
Besides, in many ways, Poirot is enabled by these tendencies, whereas Mr. Monk must function as the poster boy for a disability. This distracts from the point of the stories, which should be (remember?) the mystery.
Reason #3: The inter-war years are distinctly ripe for the kinds of cases Poirot is best at
In the years between the wars, you had British affluence attaining its full height on the wings of the industrial revolution. And at the same time, you had a culture of people who had experienced first-hand the effects of mustard gas and the zeppelin bombing raids over London. The 20s was the decade of the backlash, the collective recoil against the nightmare that human nature had revealed itself to be.
The only rational response to the horrors of World War I was hedonism. Damocles's sword had been revealed, so we'd all better drink up.
Hedonism nowadays has a much different tone. Nuclear-age, drone-strike hedonism is tempered by the knowledge that you will not see your killer. It is a hedonism stripped of humanity, its pleasure seen as an immaterial substance to be consumed alone.
But in 20s and 30s London, to embrace this fragile life meant clinging to people, which meant their successes and trespasses and big-ass emerald necklaces took on talismanic size within one's society. A size big enough to kill over.
Reason #4: David Suchet will never be topped
Poirot has had many fine portrayals, including Albert Finney's Murder on the Orient Express, which stands today as one of the great performances on film.
But no one ever "got" Poirot like David Suchet (pictured above). Every square inch of him is pitch-perfect.
A modern Poirot would be a grotesque caricature, inevitably. Ours is not a culture of refinement or subtlety, nor one that appreciates such in our crimefighters.
Reason #5: The gay issue
Of COURSE Poirot was banging Hastings. But that's THEIR BUSINESS. That's not what the show or the stories were ever "about".
If they made a Poirot series today, even if they kept it in period, it just wouldn't be sufficient to have two "confirmed bachelors" hanging out, solving mysteries. Oh no, the people just gots to know what their "deal" is.