I registered the domain for <$10, last year it was worth $50,000, and now it's worth $100k.

Oh, you meant the domain name itself as an asset, not the use you were getting out of it. Yes, I suppose it has on paper, but past performance is no guarantee of future returns. There's no reason to hang onto it. (How much do you know about domain names? Will additional TLD releases by ICANN reduce the value of weidai.com? Or is the reason it's worth $100k time-sensitive in any way? It seems like a boring domain name, why is it worth $100k anyway?)

The effect of selling it is just to increase the diversification of my investment portfolio (i.e., I would no longer have a substantial chunk of my investments in a single asset), which is a good thing since I'm risk averse, but that benefit is certainly not worth $100k.

You're taking on a huge amount of uncompensated risk here. Would you put $100k into a single stock? For example, buy a single share of BRK.A? No, of course not, even if that got you exposure to 'insurance' or 'railroads'.

Domain names are a different asset class from everything else I own, so ideally I ought to have some exposure to it.

Unless you're orders of magnitude wealthier than I think you are, your exposure to an opaque illiquid minor hard-to-price asset like domain names should look more like $10 than $100,000.