If you're well off financially and can afford to pay for a deposit box, then get one and store your seed phrase. Also, leave a note of your bitcoin investment for your lawyer to disclose to your family in the event of your passing. If you don't have the financial capacity to rent a deposit box, then you've to trust a family member close to you with your seed phrase. If you fail to do this and you kick the bucket, your bitcoin will be lost forever, except someone accidentally stumbles on your seed phrase.