THIS IS NOT A CHANGE. IT'S ALWAYS 天将降大任于是人也 IN PRINTED ANCIENT MENCIUS BOOKS AND MODERN MAINSTREAM TEXTBOOKS.
As far as I know, 斯人 is correct or more often seen and according to the tradition of Chinese words, 是人 might have been used.
This is incorrect. The proximal pronoun this mainly include, 兹（both proximal and distal in oracles)、时、是、若、斯、此. They are not concurrent but from different layers of Old Chinese. And their semantics and pragmatics are not exactly the same. Scholars agree that they are phonetically related. That being said, if we are to pick two most used ones, they’re 是 for older texts and 此 for later texts. This quote is from 《孟子》. Historically the range and the frequency of the use of 斯 had already been significantly reduced in 《孟子》 and gradually disappears after that, except that the author wanted to mimic the old works.
The debate is not on the language itself. Otherwise it’s conclusive and nothing to debate; before the end of Qing, the printed version of the Mencius book all wrote 天将降大任于是人也. If we look at the text itself, I also prefer 是 to 斯.
是 is anaphoric, refers to 舜、傅说、胶鬲、管夷吾、孙叔敖、百里奚, literally referring to previous examples but practically extended to mean anyone, thus bringing the examples and the argument. 斯 is a proximal pronoun that has a weak orientation, opposed to 此.
the national People's Education Publishing House has replaced 斯人 with 是人 in the well known sentence 天将降大任于斯人也 for the new textbook of Chinese language for secondary schools
This is not true. if you really checked the textbooks, it’s almost always 是人. Some student guides and rarely certain local versions of textbooks use 斯人 mistakenly. Those are never the mainstream or authoritative.
Central to the discussion is what we actually learned in grade schools. What's particular about it is that this sentence is so famous. We are all required to recite the whole passage and 天将降大任于是人也 appears frequently in tests as filling the blanks. Yet most people pull 天将降大任于斯人也 out of memories. So they think the textbooks must be wrong and find it hard to believe that the textbooks are correct. (in rare cases textbooks are wrong about this but it does not pertain to most people)
An interesting question would be why so many Chinese are so certain that they learned 斯人 at school. Similar phenomena also appear with the pronunciation of 说服. In all the versions of the 现代汉语词典 (this is quoted not checked myself), it’s noted shuo1. The dictionary never changes it. Then why are people so sure that they learned shui4? I have no answer to this type of phenomena. And among the discussions you refer to, many of them are actually about this.
There could be many reasons why the group memory is wrong. I guess the fact that 斯人 are used widely in other texts （especially in «论语» e.g. 斯人已逝，生者如斯） and 是人 in modern Chinese means is person has contributed to the change of people's memory about 天将降大任于是人也.
Regarding the heat, here's what I think.
Topics that completely upturned our beliefs are always easier to caught the eye. The sad fact that people don't believe it even after they read the facts and see the photos of old textbooks contribute to the heat of the discussion.
On the one hand, many articles do not list concrete evidence and post photos of textbooks. Readers believe whatever their memories tell them without actually checking the books. This makes an end to the discussion more difficult. This is not the first time this topic has been heated discussed. It's been raised up again and again and again.
On the other hand, Internet discussions are anonymous. Instead of finding reasons from themselves, it's always easier and stress-relieving to attribute to others, especially the authority.