I always figured that capital gains are taxed at a favorable rate because they tend to be owned and held by the wealthy who have the power to lobby for favorable tax treatment. I'm sure there is some theoretical justification too, but I tend to take a more pragmatic and cynical view of the world.
Intent is actually not the key. If you refer back to the three part test that you cite above, the first part is not an intent-based inquiry, but asks whether the taxpayer is involved in a trade or business. You can say that you did not intend to be engaged in a trade or business until you're blue in the face, but if you're doing something that looks like a business, the objective presentation of that activity will easily outweigh the self-serving assertion that you intended to be doing something other than operating a business (which will simply come across as an intent to avoid tax liability through clever argument).
This first question in that test ties directly to the subsection of the statutory definition of capital asset that excludes inventory (i.e., assets held for sale in the course of a trade or business) from being a capital asset. Thus, in the first instance, the IRS or Tax Court is going to ask whether you are selling Lego in the course of operating a trade or business. If the answer is yes, that is pretty much going to resolve the question because if the IRS or court finds that you are engaged in a business selling Lego, it will be pretty hard to argue that you did not intend to sell the Lego in question in connection with that business. If you can convince them that you're not engaged in a trade or business, the asset couldn't be inventory.
Also, you assert that Lego is a collectible as if it were a proven fact (i.e., "case closed"). However, what you overlook is the fact that Lego does not fall within one of the recognized categories of collectibles under the tax code and would only be considered a collectible if deemed so by the IRS under the authority granted by the tax code to categorize assets as collectibles in addition to those specifically listed in the code. The evidence that you are relying on to prove that Lego is a collectible would certainly provide a basis for the IRS to reach that conclusion, but things don't always work out in the real world the way you think they will. In reality, I would wager that your average IRS auditor is going to start from the position that what we're doing here is a toy business in which Lego is an item of inventory and that it will be a very tall hill to climb to convince him or her that it is a collectible simply because Lego appreciates over time and you did not buy it with the intent to resell in a short time frame. In my experience (none with the IRS though), government auditors tend to start from the position that you're trying to screw the government in one way or another and aren't particularly charitable or inclined to accept arguments like the ones you'd be making here.
That the safest course of action. I hold the vast majority of my inventory for 12 months or longer and buy it because I think it will appreciate, but I don't think there is a snowball's chance in hell that I could convince the IRS that I'm doing anything other than running a business.