I don't know how long it'll take for MMV to be topped, but I'd say a while. There have been a few recent sets (Helm's Deep) that have been in a similar league, but... none of them quite as good. MMV was brilliant because it was the best set Castle had produced in a while, especially for the AFOL. It was fun, well built, good value, an incredibly nice parts pack, can be added to for a layout or can simply stand alone, and something a bit different to past sets. This, as the first impression most of us got in 2008, was a pretty good start - and it has lasted, as we are still debating it four years later. I think, in many ways, MMV sparked a wee bit of a new direction for Lego - the style of set has now been mimicked in many themes - and heralded the real start for me of the current 'Golden Age' we have. The attention to detail, the great (at the time) new parts, and the fun, accepting Lego-ness of it endeared it to all.
Many bought several copies (a few a lot more than several!). I bought one, and built it, and loved it, and think it is the best set I have ever bought. I siphon off pieces every now and again for projects, but I could never bear to destroy it. If I were only ever able to thank Lego for one set, I'd thank them for MMV.
(Sorry for the rant, but any excuse to discuss the virtues of MMV I grab with both hands. )