If I claimed Amazon stock was 'worthless' and only relied on 'greater fool theory' you would say...

If I claimed Amazon stock was 'worthless' and only relied on 'greater fool theory' you would reply:

False: Amazon has a computer called Amazon Web Services (AWS). This computer has certain properties (fast, cheaper than running your own server, available worldwide, reliable, etc.) that make many people want to pay to use it. Some of that value flows back to the holders of Amazon stock.

Similarly, when people claim ETH is 'worthless' and relies on 'greater fool theory', it's important to point out:

Ethereum has a computer called the Ethereum Virtual Machine (EVM). This computer has certain properties that make people want to pay (~50M USD/Day) to use it.

Censorship Resistant - no one can stop anyone else from using it,
100% uptime,
Natively Financial - every account is also a wallet,
Atomic - both sides of a swap go through or neither,
Immutable - no one can change the rules after the fact,
Pseudonymous - the system is incapable of discriminating based on race, gender, or religion, Persistent - systems last if devs don't maintain them,
Credibly Neutral - No user or use can be favored above any other.

Some of the value of these payments flows back to the holders of ETH. If Microsoft/Azure or Google/Google Cloud have value, then Ethereum does too.

Note that some of these properties of the EVM create use cases that were not possible before - for instance, the natively financial aspect allows airdrops, a way for projects that make it big to choose to share the value they have created fairly with hundreds of thousands of ordinary, early users.

Other properties, like credible neutrality, might not seem important to some first-world users. How many businesses in countries with corruption, (or the suspicion of corruption) in their legal systems have lost out on investors because the investors know that there will be great cost, risk and uncertainty in resolving disputes?

Sure, uses of the EVM are sharply limited by the current high gas fees and environmental concerns of POW. But, POS, data shards, rollups, state expiry, and more on the horizon, what kind of value will be created and shared with ETH holders in the future?