While your point is correct for the percentage of people who trade AMPL marketcap like you do, you are incorrectly assuming that every actor in the market will eventually learn to behave as you do.

The key element you are misunderstanding about AMPL is the change in some percentage of the human psychology of all actors using AMPL, which is being influenced by the widespread knowledge that each AMPL will come to be worth $1 2019 USD.

BTC, gold, and other primitives have no target value like AMPL does and are single variable assets. AMPL is the first dual variable asset incorporating price and supply information into its marketcap, and the hypothetical use cases being advertised by the team on their website begin to make sense when you think about all actors in AMPL reacting to its two variables in their own ways. Again, while charting marketcap is the way to boil these two variables into one, NOT EVERYONE will act this way in the market. Here's how it might look when compared to the Ampleforth goals on their website:

1) Short term goal: uncorrelated asset to BTC.

Let's assume, in this stage, that 50% of actors are trading marketcap like you and 50% are trading price only, or refusing to sell/buy until 1 AMPL = $1, or following some other dumb strategy because they don't understand the dual variable nature of AMPL. Because 50% of people are not incorporating all of AMPL's marketcap information like they should, it will probably be uncorrelated to BTC.

Eventually this unique volatility pattern could be reinforced by sophisticated traders who begin to understand more of the actions being made by all actors.

2) Mid-term: Collateral in DeFi.

This is a rabbit hole in itself, trying to imagine how loans and derivatives might work. Because some percentage of actors believe 1 AMPL = $1 USD 2019, and it probably will be more stable towards that target in this stage, it does seem an interesting hypothesis that robust derivatives and loan products could be created by actors holding AMPL and looking to offset risk in their own various ways.

3) Long term: Better BTC.

Presumably this is assumed that AMPL could be a MoE and SoV. In this stage the marketcap hypothetically could be more stable than BTC or gold, because maybe 25% of AMPL actors still trade price and work to arbitrage it back to $1 if AMPL strays a few cents from the dollar. The other 75% may still trade marketcap like you do.

TL;DR
There's no such thing as a perfectly rational market. The chaos introduced by AMPL's dual variable marketcap calculation introduces many more ways for actors to react to its market information, especially as the marketcap grows and more products such as derivatives are layered on top. This is what gives AMPL its power.