Thank you for the question!
The answer is by providing the choice to the user. The user can run their own node or customize a set of open nodes to which to connect to. For example I run my own node and use it with rotki, with metamask and with otterscan (a local blockchain explorer: GitHub - wmitsuda/otterscan: A blazingly fast, local, Ethereum block explorer built on top of Erigon). That is for blockchain data.
For most other data it’s all calculated locally. For whatever we need to query external services we query multiple and try to randomize them so as to not leak data (in case user is not just privacy aware – but paranoid). For example historical forex prices etc.
Let me give you an example. Say you use zerion/zapper.
Just by you using the centralized website, they know (1) your location (2) your entire crypto networth and your (3) spending habits and possibly more depending on other factors such as having an ENS associated with the wallets you track etc.
That easily puts a target on your back if any of this data leaks, especially combined with say the ledger data leak which contains a gazillion of home addresses.
That’s the web2 way. Trust should be out of the equation. Can’t be evil is better than don’t be evil.
Which is why local-first apps like rotki are the only true dapps. All else is simply web2 centralized apps masquerading as “dapps”. Sure they are easier to start with as a user since they are centralized webapps but to what cost?
I will refer you to: Local-first software: You own your data, in spite of the cloud for an explanation of our development approach.
And just one of many questions you should be asking yourself when you use centralized apps: https://twitter.com/martinkl/status/1540971616347250688
Keep in mind. Your financial data is probably one of the most sensitive data about yourself. Protect yourself.
Finally all those centralized apps, zerion and zapper included, are completely free to use. Yet they have had millions invested on them by VCs in multiple rounds. Doesn’t take a genius to understand that the product of a centralized service like that is eventually you, the user. Those VCs are expecting a return in their investment. They did not invest out of charity.
As for us, we don’t have any investors. We also seek none. We are idealists. We have had a big initial investment by me (I have been around crypto for a while) and then it’s all organic income from donations and development grants like this one. Thus rotki is not capturable by external interests.