Well I’m afraid the one Jewish person you spoke with isn’t really speaking for all Jews. In fact there is really no official Jewish consensus there beyond Jesus did not fulfil messianic prophecies. Of course each montheistic religion will be complete (subject, usually to the the end of times) as otherwise it would leave it open to uncertainty and even more manipulation- in short it would remove its power.
If someone else came a long claiming to be a prophet ahead of Mohammed Islam would also reject it because otherwise it would deem their religion null and void.
If a new religion came along (as is inevitable) called itself Miat and said that word meant that everyone is a worshipper of God it also came along, as all religions do with a series of rules and beliefs would you think you're now Miatian not Muslim?
Islam comes with a series of requirements, a set of beliefs. Outside of those beliefs a Muslim is someone who follows Islam. If you’re saying all it takes to be Mudd sad Kim os to submit to God then why can’t a midd sad Kim eat certain things or dress in certain ways. Why do they need to believe certain things are required? If you say you need to do these things to submit to God then, by your definition someone eating a ham sandwich or any other forbidden food is not Muslim. The concept you’re talking about falls apart under even the most cursory of glances.
Of course Islam will eventually disappear, religions always do. Usually a religion will disappear either by coming into contact with a different set of beliefs, those beliefs not fitting into a changed culture or because those beliefs no longer serve the purpose for which they are designed. I would suggest, with globalisation many of these factors are currently inter playing with Islam. It might have a brief rise as it tries to rally and cling to life, but like all religions it will eventually die out and be replaced by something different.
I highly suspect the next major religion will be Earth based with rules to back up scientific analysis on how to heal the earth. Bit like the banning of shellfish in hot climates religions was designed as a public health measure, no mixed cloths as a trading standards measure, buying dead quickly etc. In fact we are slowly starting to see the resurgence of earth based religions. Traditional religions will face a dilemma of balancing the need to have sufficient numbers in order to protect themselves and not have too many followers to have insufficient resources. At this point “everyone is Muslim” will definitely disappear out of Islam.
I do always find it perplexing why followers of a religion don’t think theirs will go the way of hundreds before them.