Why blockchains like ETH, ADA, DOT, etc. can and should co-exist
The problem to be solved is to get rid of the middleman. That's the point of DeFi. That's why we have smart contract blockchains created, like ETH, ADA, DOT, etc. so we can create tokens we can trade for without the need of intermediaries.
But tribalism exists. People shit on ETH for gas fees and praise ADA. ADA is shitted on because they feel ETH 2.0 will come out and render it obsolete.
Do keep in mind that these other projects exist because they want to implement DeFi in their own way and own pace. And it's perfectly legit.
Yes, Ethereum has scalability issues right now. So the developers of Cardano just wanted to take one step ahead with their PoS implementation. Polkadot on the other hand, its goal is to bring several blockchains together to create a decentralized system between them. They have all different implementations and even implement each other's ideas. In the end, they're all trying to push forward for DeFi.
Let all these projects learn from each other in their development and progress. All of them are doing what they plan to do.
Comparing these projects on their use cases is like comparing AWS to Google Cloud to Microsoft Azure. All these are different web service providers. But if you go on a webpage using any one of them, and it works, it works, and that's the end goal.
Be supportive of the DeFi smart chain projects that have use cases, and have a roadmap of ambition and active development.