For what it's worth I think that "just a convention" might be a bit misleading, because arguably everything is. You can very well define other number systems and/or algebras in which 1/0 is a meaningful number, 1+1=0 or whatever else you can think of.

After all, every algebraic structure and every number system is "just a convention", but what follows from it is not.

Now IF you define your number system the way the real numbers are defined, and you want your usual arithmetic rules to apply, then 0.999... MUST equal 1, just as 1/0 can never be a meaningful number and 1+1 can never equal 0.

You could say that those are all " just conventions", but 0.999=1 is in no way special.

To quote the article:

However, it is by no means an arbitrary convention, because not adopting it forces one either to invent strange new objects or to abandon some of the familiar rules of arithmetic.