One can do anything.
But there are computer overheads: dont make computation too difficult.
And there are social consequences: this site includes adults and kids - so rewarding is important, and not punishing (lower scores) is also important.
But think about it. Learning how to identify is a skill learned that is not easy to forget. The purpose of iSpot is to get users to learn to ID, which is a mentor-student relationship. Once learned the skill persists. The aim is not to get people to rote learn differences. The aim is not to have beginners knowing by site all 900 bird species. iSpot is an ID tool as well, and learning to use iSpot (unless the programmers mess it up) is a learning curve, that takes mere hours to refresh even after several months or years absence.
So the real question is simply. How many IDs does one have to make to become proficient in a group? The answer is probably in the region of 200 to 1000 IDs. (for iSpot it is 500 - a silver or knowledgeable user has 500 reputation, and to get to this level, with a maximum of one reputation per ID, requires 500 correct IDs agreed by an expert, or two knowledgeables. If agreements are only by novices more IDs are needed!) Now consider - there are dozens of proficient users in iSpot in any group, so a novice entering the system will probably have to post their own IDs, so this is a very tall ask. There is a flaw on iSpot, but I have not yet seen any users exploit it mercilessly: one can post 500 observations of a common, easy to ID, species and earn that reputation. But the amount of cheating has not warranted adding a far more complex algorithm to cap or filter reputation.
Anyone who has worked on iSpot for 10 years is probably highly proficient in their group.
A bigger issue is geographical expertise. iSpot does not recognize that an expert in African birds may be a novice in Asian or South American birds. But the logic does apply: if I am proficient at identifying African birds, I should easily be able to apply my skills to American birds.
Note that iSpot reputation has nothing to do with being able to make an ID spontaneously without reference to iSpot/books/keys/other IDs tools. Reputation on iSpot is not about knowing the species, but being able to ID them. Spontaneous ID of 900 taxa in your field of interest is an entirely different ballgame: and your ideas above would apply. But memorizing 900 taxa is not necessary on iSpot - there are multiple examples of all species on iSpot - iSpot is the collective memory and the user just needs to learn how to tap into that - with the help of resident experts and more experienced users.
There is a need on iSpot though to recognize more invertebrate groups: skills in insects are not quite the same as for arachnids (or at least they are far more different than the gap between the vertebrate groups).