What asset just goes up forever? If such a thing existed, we could all be rich.
Even if Bitcoin holds the long term uptrend through the next couple halvings, it'll eventually break down. I don't see why entering a bearish supercycle (years or decade-long bear market) would change the bigger picture though. It's the same thing as the 2011 or 2014 bear cycles, just on a larger scale.
Bitcoin is provably scarce. If the demand keeps growing for such an asset them its value will also keep growing.
But I agree that it will eventually break down,
if there was an asset a lot better than Bitcoin. So far, there is none.
Key words: "if the demand keeps growing"
Even if Bitcoin is superior to all other forms of money, it isn't guaranteed endless demand. There is a limit to the amount of value that can flow into BTC, even if that limit is theoretically when every other possible type of money is valued at $0.
In all markets, equilibrium is eventually reached and overall trends reverse. Bitcoin is not immune to that, no matter how good it is.