The barriers between the Mundus and Oblivion are (or were) protected by more than just the Amulet of Kings; they are (or were) protected by a system of multiple towers and stones. The last two towers standing were Red Mountain, whose stone was the Heart of Lorkhan; and White-Gold Tower, whose stone was the Amulet of Kings. These barriers prevent the creation of a stable http://www.uesp.net/wiki/Oblivion:Liminal_Bridges.
The Red Mountain tower is of still greater antiquity than White-Gold Tower, dating back to the death of Lorkhan. Large-scale Daedric incursions were impossible due to its presence, even before the creation of the Amulet of Kings, and even during the chaos of the Second Era. But when the Nerevarine freed the Heart of Lorkhan, Red Mountain fell. This left only White-Gold Tower, with the Amulet of Kings as its stone, preventing the widespread creation of stable portals to Oblivion. Then the Emperor died, and the Amulet of Kings was stolen, taking down the last tower, and allowing the Mythic Dawn to open the gates for the invasion.
I'm sorry, but this post is fallacious and has sent the thread into quite the downward spiral due to the erroneous speculation that it fostered.
The other Towers were not and are not obsolete, destroyed, or in any other way not functioning. If
all the Towers had fallen we'd have a heck of a lot bigger mess on our hands than Mehrunes Dagon invading. Its also important to note that the Lunar Lattice plays a substantial role in sustaining the barrier between Mundus & Oblivion, though it could be argue that the Lattice has been conjoined with the Khajiit tower.
The Red Mountain may be of greater antiquity than White-Gold, but due to White-Gold's symbolic structure (that of the Wheel) it actually holds the greater sway. And again, the other Towers were still active after the fall of Red Mountain and White-Gold; however, two of the most significant stabilizers to the barrier had been removed and that's what allowed the widespread opening of gates, not a complete collapse of Nirn's structure as would occur if all barriers were broken.