Very interesting points - and I have thought about this a lot as well and I think it applies especially to the online world.

In real life (and bear with me here, because I am not a native English speaker) I see very little opportunity of using it. Even my boss wants us to call him by his first name and well, I have not been to school in England so maybe there would be someone to call Sir but I would still think the normal form of respect when you don't know someone is Mr so-and-so or Ms so-and-so.

But when I am with a dominant in real life, I have that natural urge to call him Sir - either he has asked me to address him as such or I would teasingly test the waters... like with the mentioning of 'spanking', 'good girl' etc. its something that can always be thrown into a teasing joke and taken back without any harm done.

Now online - I very rarely, basically never feel the need to call someone Sir because there is no person, no bearing, no aura. I will gladly call a Dominant 'Sir' if he so requests - but even then not always and certainly not always sincerely.

For me - yes a sub has to earn the right to call a man Sir, or at least talk to him a little until it comes from a place of wanting to give this extra bit of respect.
And on the other hand - a Dom for me has to earn this just as much. There are so many random men out there who would like to be called Sir, who have no idea what bdsm is about, who don't have any respect for women and in particular for submissive and feel it is their god given right to be respected as something better.

I call a Master Sir, when that common understanding is reached that we mutually respect each other and wish to interact in a friendly manner, with him as the alpha in the conversation... because I, too, think it can loose some of its meaning.
I also never call a man Master online - well, I used to when I was a lot younger and hadn't figured out how things worked out for me personally, but now Master is reserved only for the real life man I submit to.