Modern crypto algorithms are decay-free. They may still get weaker because of improved math, or quantum computers, but never because of increased computing power. That was an artifact of too slow computers at the beginning of computers history.
Modern algorithms are replaced mostly because the alternatives are easier to use, faster, or more flexible in some ways. With some huge emphasis on "easier to use" because that means "more secure" on practice.
All the time is a bit of an exaggeration. Some algorithms are broken very fast, others slowly accumulate partial attacks until people don't trust them anymore. Those last ones don't normally get completely broken¹. I think the only exceptions were the shorter key lengths of RSA².
1 - But you will find many examples of algorithms with less than the modern strength parameters that were broken by the mix of faster computers and partial attacks.
2 - But by the time those were abandoned people were mostly using even shorter keys that wouldn't suffice by today's standards even without any known attack.
Modern algorithms are replaced mostly because the alternatives are easier to use, faster, or more flexible in some ways. With some huge emphasis on "easier to use" because that means "more secure" on practice.