I'm not sure what you were seeing, but I spent a solid hour playing with my own data without (yet) creating an account.
Aside from that, this kind of low-effort dismissal ("has/lacks X, closed the tab") plagues a lot of Show HNs, and it's not constructive. If you really feel so strongly against signing up, it would be more constructive to explain why you feel that way and suggest ways OP could improve their project. As it is this comment doesn't contribute to any meaningful discussion, it just comes off as rude.
I was able to put my data in. The simulation predictions still showed $1m+ retirement data.
Also, I feel strongly against signing up for stuff because I already get tons of spam marketing from sites that I have signed up. I also am very protective over who knows what about my money.
For what it's worth, data already stays client-side by default. If you upgrade and enable saving, you can choose from localStorage only, cloud sync, or manually importing/exporting JSON files.
Aside from that, this kind of low-effort dismissal ("has/lacks X, closed the tab") plagues a lot of Show HNs, and it's not constructive. If you really feel so strongly against signing up, it would be more constructive to explain why you feel that way and suggest ways OP could improve their project. As it is this comment doesn't contribute to any meaningful discussion, it just comes off as rude.