Your reasoning is correct, but it has the consequence of keeping the cryptocoins away from general public. Until the specialized hardware (including high-end GPUs) is required to mine the coins, we will very rarely get average Joe to try the system. Imagine if every computer and every mobile phone would have an equal chance to mine something, we would have dozens of millions of new users in no time. So there's a catch - either you want stability with ASICs and great difficulty, or you want mass adoption, you can't have both. Claims that botnets would be a threat make no sense to me, why would we mind few botnets even with 100K machines under them if we have 100M machines successfully mining coins? Botnets can not have double digit percentage of machines polluted, they would be detected since it's obvious they are working in the background, so let them mine their few percentage of mining power.

Problem is that it's very difficult to find the hashing algorithm that would be really ASIC resistant, not to mention GPU. For instance, Primecoin which included large integer algorithms that should be difficult for GPUs was soon transferred to GPU mining. All scrypt algos are prone to GPU mining. I tried to find some hashing algorithms that would favor RISC processors, since they dominate modern smartphones, but I've failed to find any. If anybody has some info about such algos it would be very nice to share that info here.