The basic idea is that the price of Bitcoin is mainly influenced by the supply cycle, assuming that demand more or less stays the same. So scarcity dictates the price range, rather than usefulness.
This bold part is the problem with this theory, it is a wrong assumption. It can work for something like a popular stock, a precious metal, etc. These things aren't being adopted, their demand is more or less the same as it has ever been.
On the other hand bitcoin adoption (ie. demand) is increasing so the theory has to become a lot more complicated to account for that increasing demand.
I must admit that I am one of the doubters, especially because I see the usefulness of Bitcoin (not Blockchain!) lagging.
It is not "lagging". Bitcoin is one of a kind and in that sense is offering a very unique utility as a censorship resistant currency that could be used globally with highest security available while being very fast (your transaction reaches the entire world in matter of seconds and is settled on the ledger in 10 minutes on average).
You have to keep in mind that "usefulness" is not defined as creating vaporware such as what tokens are doing these days fooling people into thinking it is "usage"!
For coins like XLM or XRP, you can argue that they are useful, because they actually support payments across systems and country currency borders already. But Bitcoin transactions are slow, transaction fees are relatively high, and the fixed block size limits scalability.
First of all when you compare two things you have to compare them correctly. For example a useless blockchain that nobody uses will obviously process 100 tx/day easier than a useful blockchain that handles 300k-500k tx/day. Secondly speed is not about how fast a tx goes into a block, it is about security that the said block offers. 1 confirmation in bitcoin that takes 10 minutes on average has a security that is leaps ahead of 100 confirmation in an altcoin's chain.
Finally it is about centralized versus decentralized. Both shitcoins that you named here are fully centralized and are in control of the company that has created them. Exactly like PayPal so if you want to compare these two you must compare them with PayPal and Visa not with bitcoin.
So instead of the currency becoming scarcer with each halving, who guarantees that miners and users won't simply move away to more useful coins over time?
Users don't care about circulating supply if something is indeed useful. I promise you that if some day an actually useful alternative cryptocurrency came along we would all be using it regardless of its price and supply.
(both suppliers and consumers have multiple choices).
Sadly we only have one choice when we are choosing in category of secure decentralized currencies.
If we were looking in the category of centralized pump and dump coins with potential of big profit in short term then we have about 20k-50k choices.