Grinding hard on Run Your Own Mail Server.
Modern DKIM uses 2048-bit keys. With current mathematical understanding, they are not brute-force breakable before the expected heat death of the universe. Modern cryptographic algorithms don’t fall to brute force, however. Mathematicians nibble at them, discovering weak point after weak point until, eventually, someone figures out how to break them in a reasonable time. Computer speed might not be accelerating the way it did a couple decades ago, but processing power is far more accessible than ever. Every year, any definition of “reasonable time” encompasses more and more processing power.
Will your key be broken? Probably not.
Is your organization a target for intruders? Does you handle money or personally identifiable information? Are legal stormtroopers likely to loom over your world and blast it into a billion shreds? If so, rotating your key every year or so is a respectable item on the list of things you do to convince auditors that you’re taking sensible precautions.
You still have time to sponsor this. Some time. The start of a book is slow, but the end is often an explosive implosion.