Peoples' beliefs were malleable and coercion was just one strategy out of many that persuaded people to change beliefs. Given that Muhammad's military prowess and his treaties with various groups are well-attested to, surely it makes sense that it was his personal influence that increased the reach of the early Muslim community? If early Islam was as set in stone as you suggest, why would anyone apostatise?

Anyway, you are analysing the issue through a religious lens rather from a critical historical perspective. Historians of religion would approach the argument quite differently. Take for example this:

Actually in Islam, these rules are absolute. In fact, this is central to Islam. This is one of the most important concepts of the religion. The fact that rules are absolute and set by God have always been a part of Islam. This seems like a pretty huge misunderstanding. Religious rules/laws in Islam are set by God himself.

Many people believe this, yet it is surely clear that not everyone does? This doesn't explain at all why someone would abandon the early Muslim community in the 630s.

Well what you said was that Muhammad never said anything about who should succeed him, when the vast, vast, vast, vast majority of Muslims (both Shi'i and Sunni) would say that this is obviously false. You have focused on an extremely obscure opinion in Islamic scholarship. Not even close to a mainstream one.

It doesn't matter what modern Muslims think, we are trying to get at what a Muslim in the 630s thought, for which we have very little evidence aside from the Qur'an, which obviously does not provide evidence for the beliefs of everyone in Arabia.

It's extremely stupid and simply wrong to assume that the only reason that people followed Islam is because otherwise Muhammad would have punished them. Muhammad did not have enough power to do so for the vast majority of his life.

The other person was clearly talking about Muhammad's influence at the end of his life, which was undeniably great as he seemingly had gathered together many Arabs from across Arabia under his banner, so yes, it is entirely justifiable to say that Muhammad played a major role in including many people into his nascent community. Anyway, he/she said nothing about the Prophet forcing people to be Muslims, only that Muhammad's interpretation of religion became popular due to his actions and that Muhammad's movement possessed a great deal of power, which can enforce his ideas on others. I mean, his stance against paganism and people who were against the umma is widely known about in from later sources, so force definitely had a role in the development of Islam. Muhammad was a charismatic man and a great politician, but it is too simplistic to say that everyone became devout upon hearing Muhammad's message and that this was why they acted in a certain way.