Because blockchain data is public, anyone can analyze historical transactions. You don't need fancy tools. You can figure anything out just looking at a chain explorer website like etherscan. The problem is that this is a very tedious process. It's much easier to use a tool that provides data visualizations, supports queries, and maybe even leverages AI.
The tools that are available with these features today are typically paid and target DeFi. The people who are making money on DeFi arbitrage, front running, and otherwise leveraging blockchain for maximum profit are more than willing to pay for the best tools.
However, DeFi isn't the only use case for blockchain. Whether we're talking about video games, supply chain management, private health data storage, or anything else, there's a need for good analytical tools.
To be clear, there are plenty of tools that analyze and broadly compare dApp usage, but few that can be adapted to analyze any dApp's application-specific data. If I were to build this, I would offer it as a whitelabel dashboard that can be customized and offered (free or paid) to a dApp's community. If the company behind the dApp sees value, they can pay for it themselves and possibly choose to offer free access to anyone.
The needs of each dApp will be different, so there's opportunity for a lot of custom work as well as paid access for premium features like alerting.
P.S. Ethereum seems like the obvious place to start, but personally I would choose a newer chain to validate the idea. A smaller community will be more supportive, and you'll benefit from those vocal early adopters as you expand the service multi-chain.