I just find it difficult to believe Supply and Demand applies to some alts that are heavily manipulated.
Agree. Small alts are heavily manipulated and it is harder to trade. But top 100 alts are all good opportunity. Unless there is a blackswan (even Forex has blackswan), trading with altcoins give you more profit because of more opportunities and lower leverage level.
The Supply and Demand theory is useless in crypto market. Price is not moved simply because of supply or demand. If it is moved like that theory, price of bitcoin or altcoins don't have over bullish or over bearish times. Supplies don't change with some cryptocurrencies, they have fixed total supplies. The demand is only presented clearly in bear market. In bull market, demand is faked by hypes, greediness and leverages.
If you believe in demand in bull market, you can accept overvalued prices and that is not good. If you accept it and use leverage, you will be demised when fake demand vanishes into the air.
You are kinda misunderstand the position. Supply does not mean that the current supply of a coin but actually, a number of coin which sellers intend to sell. By that I mean, selling bitcoin at $10000 is a totally different supply with selling bitcoin at $60000. When an appropriate supply meet a fair demand, the price will kindly be stable. And as you can see on the chart, because of the hype of cryptocurrency, the current demand is always higher than supply which force the price to go up
The fixed number of cryptocurrency is also an interesting point. It destroys the inflation which is the intrinsicness of cash