First, the 51% attack is hardly or nearly impossible to happen. In the past, some big mining pools have dominant hashrate but did not make 51% attack. They were aware that if they do it, it will be only bad for everyone in Bitcoin market, includes Bitcoin miners so they don't do attack to harm themselves.

Second, Bitcoin community, miners, nodes can react quickly with very good automatic bots nowadays.

Cost of 51% attacks is very high and benefit is very low.

Lastly, you understood incorrectly, that even 51% attack happens, it won't steal all your bitcoin if your bitcoin is from a past transaction that is very deep under the ground, with many confirmations like thousand confirmations or more, 51% attack won't affect you. Because basically with one more confirmation, your bitcoin becomes more safely.

How many bitcoin confirmations is enough
https://jlopp.github.io/bitcoin-confirmation-risk-calculator/
https://web.archive.org/web/20181231045818/https://people.xiph.org/~greg/attack_success.html
https://howmanyconfs.com/