I've seen it many times in maths arguments. Unfortunately, that doesn't make it any less incredibly wrong.
You see, infinity doesn't have an end. You can't put a one after the infinite amount of zeroes ends because, welp, it's infinite. The one never comes, the curtain never drops, the game is never concluded. In other words: 1-0.(9)=0.(0)=0
Actually there are contexts where you can do this. There's nothing wrong with the ordering .000...1 as a map from the omega ordinal to {0,1}. The problem is that this isn't a meaningful real number.
That's right, but my point is that it's all the difference between .999... and 1. If we accept that .000...1 is equal to 0, then .999... is equal to 1.
No. .000...1 is not equal to 0.00...1 is just not a valid representation of a real number. Think about it this way: When I write something like .123 I mean 1/10 + 2/100 + 3/1000. Now, if I have a non-terminating expansion say 3.1415... then this means i have 3 + 1/10 + 4/100 + 1/1000 + 5/10000 which is a convergent series. Now, ask yourself, what series is represented by .000...1? There isn't any.
If you are still having trouble with this idea it might help to read an intro real analysis textbook which should make clear what is going on. The construction of reals from the rationals using either Dedekind cuts or Cauchy sequences will both make it clear what is going on.