Food for thought:
1000 hp guy w/ 6 armor = 1360 ehp --> deso --> 1000 hp, 73.52%, 26.48% reduced
1000 hp guy w/ 12 armor = 1720 ehp --> deso --> 1360 hp, 79.07%, 20.93% reduced
1000 hp guy w/ 18 armor = 2080 ehp --> deso --> 1720 hp, 82.69%, 17.31% reduced
1000 hp guy w/ 24 armor = 2440 ehp --> deso --> 2080 hp, 85.24%, 14.76% reduced
So yes, it does get less and less effective, but even with 24 (TWENTY FOUR) armor, deso still shaves off 14.8% of their physical ehp, compared to 26.5% from a guy with 6 armor (SIX).
I think 15 armor is more typical of a carry. Deso vs a 15 armor enemy:
100 hp guy w/ 15 armor = 1900 ehp --> deso --> 1540 ehp, 81.00%, 19.00%
Now, the thing is, this math only works out if you attack them when their hp is full. Otherwise you'll just have to go with a damage amplification formula.
Since these ehp calculations are all percentage based, reducing 6 armor from a 2000 hp enemy will shave off twice as much EHP as that -6 armor would to a 1000 hp enemy.
Conclusion:
Desolator still retains its effectiveness even if it does get somewhat weaker as enimies bulk up on armor. Desolator gets better the more hp enemies have, and weaker (somewhat) with the more armor they have. Because enemies will get both, more hp AND more armor, it is sort of a push and pull on deso's effectiveness.
Personally I think deso is better than people give it credit for.