This isn't necessary to maintain value (think of a rare painting), but it is important to provide liquidity when you come to sell, so that you don't have to wait weeks for someone to buy your bitcoin. Bitcoin has plenty of liquidity - no shortage of buyer and sellers. Coinbase alone trades ~$1 billion of bitcoin per day.
You're correct that the hash-rate is related to block subsidy + fees, but it's only one of the variables. Cost of energy for miners is an equally import factor (continuously decreasing as miners tap into wasted energy around the earth, and without geographical constraints), along with efficiency of the mining HW.
Bitcoin averages around 2700 transactions per block which currently provides miners with total reward of 6.25 + 0.3 (tx fees) BTC = ~$334,050. If we suddenly went to fees-only today, that would be ~$123 per transaction. Considering that this fee is independent of transaction amount, it is actually very cheap for large values of money (e.g. >$1M) considering it covers the cost of moving the bearer asset around the world within ~30 minutes, and also covers the cost of protecting your value against debasement and theft for however long you had it stored for (often years).
I'm not sure how you came up with a % cost per year - the cost of storage is a one-off tx fee at purchase and sale (just like physical gold, but bitcoin transactions are generally cheaper and independent of the transaction size)