Sure, but when speaking in dollar terms, your asset appreciation models (stock indices) represent tangible businesses with assets and revenues in real dollars. That is, if I buy stock in X company, I (theoretically) own a certain share of the total assets of the company. A company is relatively easy to value in terms of real dollars because you can use its total assets, debt, revenue, projected growth, etc. to provide a rough score of its worth. You can assume that when a stock's value increases, the company generally has improved on these metrics in some way.

With Bitcoin, your "asset appreciation model" relies on people continually buying bitcoin at a larger rate to usd. There is no underlying asset (a business) that has intrinsic worth that may drive the price. You could argue that there are some analogues, for example, the mining hash rate or the strength of the blockchain, but the actual value of these is extremely hard to define. That is why Bitcoin may be in a bubble, people may be overestimating how these intangibles may be affecting price; it is hard to say what the value may truly be.

I am not authoritatively stating that we are in a BTC bubble, just that I would be very cautious with bitcoin and think a bubble is likely. Nobody knows what will happen, but we certainly can't say that Bitcoin isn't in a bubble (as you seemed to originally suggested), and in my opinion bitcoin resembles a bubble, though I respect that it may not to you.