NTG4.5 was released in 2010, the last version of the maps (so far) is 2020, so that's 10 years later, not too bad. But yes, it's quite erratic.
I think that the issue is possibly that demand dropps off after the first 3 years of free updates, and then drops off again when the car gets older - at ~£150 per update not many owners will continue updating, especially given the limitations of the COMAND navigation (no speed cameras, and no live traffic on older cars now that Vodafone switch off their 3g). It gets to a point when the main commercial activity is that of cut-price pirated discs and hacked PIN codes on eBay, which obviously generates no income to MB.