No, it's a MATHEMATICAL DEFINITION. We aren't assuming anything, it has nothing to do with philosophy, this is how math is defined to work.
By definition, it is a metaphysical assumption. You've presented a false dichotomy and betrayed your own PROFOUND ignorance. Philosophical, fundamental assumptions - also referred to as "axioms" - underlay ALL mathematical understanding. Your contention is comically wrong - another example of a math fanboy who sucks at logic and fundamental understanding.
Again, you have it all backwards, philosophy is totally irrelevant to the issue.
Do yourself a favor, kid. Look up the definition of the false dichotomy fallacy. Also look up philosophy of mathematics and maybe that will help a bit, because your arrogant presumptuousness is honestly laughable.
When you say you don't believe that 0.999...=1, that is exactly as valid as saying you don't believe that a full house beats three of a kind.
I phrased this that way on purpose - because I'm not an ignorant twit. I do not know with certainty that the statement 0.999...=1 is not true, but I believe it in a logical way. "Belief" does not necessarily oppose rationality - it is not necessarily emotional, as your excessive television watching may have lead you to believe. I was open to rational mathematical rebuttals that apparently are not to be found here.
we defined infinity
Definitions do not make truth. Truth, hopefully, makes accurate definitions. Truth is objective.
You can't argue that three of a kind beats a full house, you could only make up a whole new game where that would be true.
Except the rules of the game exist objectively and we either understand them accurately or we don't. Your understanding of infinity may or may not be accurate. Either one is a metaphysical possibility.
Your lack of understanding so far makes me believe that you aren't going to understand this rebuttal of mine either, and will therefore continue to "know" (haha) that you are correct, so I will leave my account here for the day so you have a chance to read this because I love snuffing people out like you, and then I will delete my account which I do regularly anyway.