That’s called indifferentism. If Muslims worship God, instead of Muhammad’s mythological Allah, and God accepts their worship, why don’t you convert to Islam?
That’s Jean-Jacques Rousseau’s position: “that one may serve God equally well by remaining in whatever religion they were raised, or converting to any which pleases them more”. Needless to say, I believe I’ve made my opinion on that matter known.
And I won’t even get started on the pit-spawned creed that is mainstream Sunni Islam - no doubt there can be nice Muslims, as I strove to be during my years as one - but not if they practise the religion faithfully (thus I left the religion, as it was either be an observant Muslim stuck in a cultural enclave withdrawn from mainstream society, or not be a Muslim at all) for the religion demands the subjugation and mistreatment and mistrust of all kuffar: it says many times in the Quran, “…and you shall not take unbelievers as friends or protectors, not even from the people of the book,” and there are many “authentic hadith” (sources of definitive oral law recognised by Sunnis) that make such remarks even more restrictive, and abundant.
No doubt there are cultural Muslims, just like cultural Jews, who identify as Muslim but do no more at most than fasting and doing the meaningless ritual prayer during Ramadan, just like some Jews don’t eat kashrut outside of Passover, and celebrate Yom Kippur and Hannukah, which beyond you could never tell they were a Jew.
Indeed, I read at one time, I do not know the reliability of it, a poll conducted among the American Jewry, and it claimed that around 30% of Jews identified as Atheist or agnostic as it pertains to God.
Digression: I studied under a Shi’i professor who was like this: he did the minimum mandatory so no one would look at him askance or report him to the government for neglecting his din, but was no more Muslim than you or I beyond that. It seems that this kind of “Islam” is much more prevalent amongst Shi’i, with deep roots in history to the triumph of the Mutazilites (rationalists) on the Shi’a side and the Asharites (occasionalist anti-intellectuals) on the Sunni side, and the continuing importance placed on 'aql (reason, rationality, logic) and the malleability of Shi’i compared to Ahl al Sunnah to adapt to the modern world due to the clerical structure and the perceived continued divine mandate and ability to re-interpret the backwards, dead, and bellicose texts at the heart of Islam in a much, much more metaphorical manner.
If Islam survives through any method other than conquest, it must reform, and the Ahl al Sunnah can not reform: a Sunni (the ridiculously legalistic mainstream branch of Islam that I belonged to, specifically Hanafi Athari Sunni - which is a rare combination, nearly contradictory, almost like saying a “Calvinist Catholic”) idea of reform is Salafism (even more strict, pseudo-patristic) or Wahhabism (even more strictly strict, pseudo-patristic and political) or Qutbism (violent and political - Osama bin Laden was a Qutbi), so, if Islam does reform, it will survive in the guise of a Shi’a (the most rationalist of all Muslims), Ibadi (the most interpretative in a symolical-metaphorical manner), or Sufi (the most mystic), or syncretic interpretation (as Islam is already ridiculously syncretic, combining elements of Judaism, Christianity, Sabaeanism, Mandaeanism, Paganism, Zoroastrianism, and in Sufism, even some truly Eastern religion with Dharmic and Daoist influences).