Not necessarily. Just because they were Gods doesn't mean no one would challenge them. They were challenged by the Nords and the Avakaviri at various times and had to use their God powers to protect Morrowind. Even if the Tribunal did take the Imperiel City, they would still face outside incursions almost constantly....it would be a never ending pretention if if they did win. Look at the Nords for exmaple, Before Kyne gifted men with the Power of the Th'um the Nords still waged war with dragons and were still shouted down, and yet the Nords just kept fighting with the dragons, and just kept getting shouted down for many many mnay years....it wasn't until Kyne finally took pity on them that changed that.

So I would venture to say even if the Tribunal held the Imperiel city for that long, The surrounding countries would still attack it and try and take it...they would eventually succeed around the time of the 2nd era 882 when the Tribunal no longer has their powers to use against the invaders....at that point it becomes a numbers game, and no matter how fortified the Dunmer would have it, when the Nords, Argonians, Khajiit, Bretons, Redguards, Orcs, Wood Elves, all come knocking, without their god powers, there is no way they could hold them off for any period of time....

I was thinking more along the lines that they held it for 250-270 years, and then eventually are forced to withdraw because they are losing their powers to Dagoth Ur and can no longer hold on to it...then the warring factions begin fighting over it, with Morrowind kinda pulled out of the whole affair to some degree.