The original idea with Addons was that they were supposed to be something you could work on after you had your Diamond. It was a concept aiming to please those who has been geocaching a lot, for a long time. The design was even to require a Diamond to show them.
When we started detailing it out more and more we figured that some Addons will be fairly easy and others more or less impossible. The original idea was definitely aiming more for impossible than easy, therefore we feel that the mentioned Addon actually has the level of difficulty we anticipated.
It's definitely not meant that everyone should get every Addon, and even more so not within a year or two.
Personally I (and the group I geocached with) aimed for > 100 finds last Leap day, and we also made sure to log D5, T5, some different sizes and types (though not all).
This Leap day I (and a similar group) will aim for full D/T, and add a few more types and sizes (Large, CITO, Virtual, Challenge).
Next Leap day in four years will probably either be 400+ finds, or 11 types. The Leap day after that, eight years from now, will be the other. If nothing has changed by then. This is the way the Addons are meant. Years of planning.
I don't expect that too many have logged 150 T5 Geocaches with the Climbing attribute either for example. It's also something that is meant to take years for most.
And I know I am repeating myself here, but the idea with the Addons was/is to have something after the Diamond has been reached. Maybe even after the loops on that Diamond have been capped. Though some Addons turned out to be quite easy. You can see them as teasers to the rest. To conclude, they are meant to be piss-hard.