Some people will think that being backed by nothing is a negative. Actually, that is irrelevant. Only scarcity matters for the price, and Proof of Work cryptocurrencies have succeeded in making currencies that become more scarce over time - some faster than others. The cost of mining rigs helps ensure that scarcity.
I partly agree with the hypothesis that Bitcoin's "not backed by anything" - it's at least not a backing like the gold standard where you could exchange a Bitcoin to [random other asset].
But: Only "scarcity" matters? We have to look closer into the meaning of the word "scarcity". At least, a "limited supply" of coins and mining equipment isn't the determining factor - and that's what most people associate with scarcity. You can create a second Bitcoin with exactly the same or a even more restrictive reward schedule and chances are it won't have the same value (there were some altcoins that tried that and miserably failed, even if we ignore the 51% attack risk).
Thus, it is important that there's an ecosystem providing value to the network. Often the word "network effect" is used for that - more users willing to buy or accept it, more service providers (which make the currency even more useful) result in a higher value. And that's what the OP is referring to. I think it's a complex system of value creation, with lots of ingredients, that make Bitcoin special, even in comparison to similarly designed coins like LTC or XMR.
Of course, if you're using the word "scarcity" correctly incorporating demand into the equation, then the "ecosystem" becomes part of the demand side.
The respond is actually pretty simple, just ask them to show you a real use case of shitcoins like ethereum in real world that is not for gamblers and speculators. There is none!
Mostly correct, although I would argue that algorithmic stablecoins like Dai do have some use - for example as a means to save money in countries with an unstable currency like Argentina. But yes, most of the DeFi space is simply "trading a certain kind of ETH/whatever transaction for another one and whose transaction gets more hype wins!".
And NFTs can be done with any cryptocurrency, including Bitcoin.