It's true that kobolds don't last too long as respectable enemies, but I will point out that a level 3 solo (750 XP) is worth more XP than a level 20 minion (700 XP). So if the DM is willing to restat monsters, the same legion devil legionnaire that almost took out your entire party on their first adventure could still be a component of a challenging encounter in the early epic tier. It's more likely that a monster might go from being a normal monster (level 3 brute) to a minion (level 11 minion), but that still provides a +8 boost to its level, greatly increasing the longevity of that monster type in the campaign.
You are right and this is exactly the way to deal with it for me. The first page of this discussion focused on the level of the human opponents yet in 4e the way of representing toughness is by xp - since level is such an abstract number for monsters. So rather than saying that in general city guards are 5th level or lower (as I would have in 3e and earlier), saying that a guard is up to 200xp gives much more flexibility at upper levels when using minions. In fact, as a player I could accept a mix group of 5th level soldiers and 13th level minions without worrying about the numbers
However to add to an argument some others have already used. The A beats B which beats C analogy is too simplified I would say. In a typical adventure at low level its more consistent than that. By the time you have waded through a kobold adventure for example its more like:-
enc 1 adventurers struggle to beat the kobolds
enc 2 adventurers struggle to beat the kobolds
enc 3 adventurers beat the kobolds
enc 4 adventurers easily beat the kobolds
enc 5 adventurers easily beat the kobolds
So by this point the PCs have set an expectation that they have overcome the general threat of kobolds, elites and bosses being exceptions. Now you could argue that in another part of the world kobolds are harder but that to me would stretch it. It would be like saying wolves in Canada are harder than those in Russia. So, at high levels I would use the minion rules to bring back kobolds but never to create a challenging encounter. As an easy encounter or maybe a filler to something much harder but not as a challenger, again solo and elite excepted. To do so would be to change the proven expectation that the players are harder than average kobolds
Until this thread I was struggling still with the use of minions and I now have some good ideas. I never bought the whole how to have a bigger fight argument since in 3e many of the fights were mass brawls with multiple opponents anyway. However minions providing a way of using lower 'level' creatures as fodder in higher levels fights (and occasionally lower level fights too) is a good use