I presume that MS will only know the number of users by 365 licenses, sales numbers and those enabling updates. They can only guess at the actual number, as there will be huge numbers using it with Runtime that don't know they are, or maybe care. My understand is that it is the largest database development system in the World. Mainly because there just isn't anything out there with the full feature list of Access. Or anything even close.
But it is always badmouthed, usually by people without a clue. Only earlier this year a bright developer guy, bit of a propeller head said that Access was only of any use to create a preliminary application. After that the actual application would be created in another system. First I told him that was nonsense and then I asked just how many people he knew who had spent weeks creating an Access example to then throw it all away and then rewrite the whole thing. Following a lengthy silence he then became a bit nasty. Basically on pushing him, he hadn't a clue about Access and I doubt if he'd looked at it for longer that an hour, ever. This ridiculous statement was being bandied about 20 years ago and I cannot understand it is still believed. Do MS ever actually promote Access? Or ever attempt to counter ludicrous assertions?
