Bitcoin is safer than gold for long term storage. At least in some respects. So what is your prediction for Bitcoin in 20 years? And why will it be less safe than gold stored for an equal amount of time?
I agree with your opinion that Bitcoin competes more with cash, checks and credit cards than gold, but bitcoin also has strong points in terms of storing value. For example, you can create "paper wallets" where the keys are never stored on a computer so it cannot be stolen by hacking a digital device that you own. Or you can create multi-signature wallets which allow you to split the keys between multiple locations. So if one bank vault is robbed, your coins are not at risk and can still be moved to a new secured location at your leisure.
Once the robber breaks into the vault, your physical gold is gone. But Bitcoin is like "ether gold" where you can divide it into 3 or more parts where you need to combine 2 or more of these parts before you can reconstitute and move the original "ether gold".
The gold you left for your great grand kids is not protected by advances in cryptography.
And also: you can verify that your bitcoin is exactly where you think it should be from a remote location. Gold in a vault requires you to go to the location where it is stored and then inspect it in person. And this assumes that your gold investment for your grandkids is something more than a piece of paper promising gold on demand in the future. Whereas, you can safely view your bitcoin exists where you expect without creating any additional risks to its safety.