No idea. Seems a bit unfair, don't you think? Which is why I don't take it on myself to play god and assume which Muslims are "true" and which are going to hell.
I said "Sincerely saying the shahada and following the five pillars to the best of their abilities". Why did you miss out that essential element of my post?
I agree that Islam is rife with sectarianism and many Muslims seem overly concerned with denouncing other Muslims as "Not Muslims".
This idea that if someone only do the five pillars is over simplification of Islam.
The Quran has far more teachings to be followed.
If one follows the 5 pillars and still be a follower of a sect, according to Quran, he is committing Shirk:
"Turn ye back in repentance to Him, and fear Him: establish regular prayers, and be not ye among those who join gods with Allah, Those who split up their Religion, and become (mere) Sects,- each party rejoicing in that which is with itself" 3:32-33
As for those who divide their religion and break up into sects, thou hast no part in them in the least: their affair is with Allah: He will in the end tell them the truth of all that they did" 6:159
Now, in practice, most of the Muslims, are dividing themselves. The Sunnis separate themselves from Shia, and like wise Shia separate from Sunni. It is not as simple as claiming "I am just a Muslim". Some came up with another sect called Quranism, which is just another separation.
Or for example according to Quran, we are not supposed to differentiate between God and His Messengers, or we will be considered as disbelievers:
"Truly those who disbelieve in God and His messengers, and seek to make a distinction between God and His messengers, and say, “We believe in some and disbelieve in others,” and seek to take a way between they are indeed the true disbelievers. And We have prepared for the disbelievers a humiliating punishment"4:150-151
Almost all Muslims differentiate between God and His messengers.
These teachings of Quran are forgotten, and most Muslims think they are supposed to differentiate between God and Messengers, whereas originally Quran taught otherwise.
So, No, it is not unfair that Muhammad said majority of Muslims will not end up in heaven. It just seems, Muslims made a new religion that ignores many important things in Quran, and simply focuses on the 5 pillars, which are easy to do. Anyone can say the Salah. Most people can fast. But these are the sufficient essential things. These are outward acts. What is important is within.