Gas limit per block is a dynamic value. This value is increasing every year. Is it theoretically possible that such a value can somehow decrease?
The block limit can increase or decrease, depending on what the miners choose.
The miners have a limited ability to adjust this value in order to help (or hurt) the network.
This value has been changed at times of high network stress, such as when there were too many transactions to process and also when there was an ongoing DDOS attack.