(This may appear twice; the forum software appears to be misbehaving. If so, my apologies, and I'll try to delete exactly one copy.)
Three preliminary points.
First, I didn't write ArM4, although I was a major contributor.
Second, anyone who hasn't read my original essay should buy Sub Rosa 12 and do so. Support the zine! 
Third, I don't think it's useful to frame the discussion in terms of increasing sales. When comparing to White Wolf, for example, you should remember that ArM didn't sell as well as the World of Darkness when it was published by White Wolf, using the same techniques as they used for the WoD. It's also worth bearing in mind that WW have stopped producing conventionally printed products; while I suspect that WoD products currently sell more copies than ArM supplements, I wouldn't bet much that I was right. All the rumours I've heard about RPG sales suggest that ArM has come through a period of catastrophic decline in the RPG market relatively unscathed. ArM might actually be doing as well as it can in the current market.
So, let's talk about onboarding and scaffolding. I agree with Timothy that these are important things to take from narrativist games, and with Arghmark that the elements he mentions help with this. Roughly, I'd say that art and fiction primarily help with onboarding, while introductory rules are scaffolding.
An important part of onboarding, possibly the most important part, is telling players what the game is about. What will they be doing? What sort of characters are they playing? What is the world like? You need to supply the characters' common sense. This is where licensed properties, if done right, have a massive advantage. "You are playing in Middle Earth. You are heroes fighting against the Dark Lord Sauron and his agents." "You are playing superheroes. You wear spandex and fight crime." "You are playing in the world of a Game of Thrones. There will be treachery, murder, and incest, possibly all at the same time." Later on, of course, you can add nuances to this, but for beginners it's a great help. Some independent games also have an easy in for this. "You are vampires in a darker version of the contemporary world." "You are warriors piloting giant walking robots."
The further removed from experience the game world, the harder this becomes. Ars Magica is a long way from the experience of its players, so onboarding is hard.
However, this does not mean that all games should be close to experience. Exotic and unfamiliar settings are fun and interesting, and can raise interesting issues and stories that just don't fit in a game based more closely on what we know. It does mean that you have to work harder on onboarding.
Scaffolding is also important, because no-one can learn all the rules at once. It really should be easy to sit down and start playing a game; half an hour's preparation is an absolute maximum. Ars Magica is really not good at this, although I would say that Tenra Bansho Zero is actually worse, at least for a western audience. (It doesn't help that TBZ is designed for short stories, not long arcs -- I suspect that you would spend longer reading the rules and background and designing a character than you would spend playing. This may have been fixed a bit in the English translation, but I haven't read that yet.)
However, I also think that the scaffolding should be an integral part of the game. The scaffolding should lead organically into a long-term campaign, with a sensibly generated character, and not make you want to throw away your first characters and start again now that you understand what is going on. This is also hard. What's more, I think you need to design the full rules with the scaffolding in mind to make this work. I don't think it can be done for Ars Magica 5, and I think a version of Ars Magica it could be done for would be very different from the versions we know.
That said, there are halfway houses: introductory adventures are a good one. The players might well want to make new characters afterwards, and the SG has to put in a lot more than half an hour of preparation, but you can get players involved quickly, and introduce elements of the background one at a time.
So, in summary, these are difficult problems for games like Ars Magica. You can make them easier by having simplistic rules and familiar settings, but even then they are not trivial. And then you have to remember that the original version of D&D, which started the whole hobby, was, by all accounts, really bad at both. D&D wouldn't work in theory, but in practice, it was a great success.
