30 years of experience writing bad code, with no effort to improve, doesn't make you any good. You need to right attitude and humility to become good.
Some of the worst programmers I have ever worked with had 30+ years of experience. They basically spend all of their time fixing bug after bug in a never ending cycle because the software they produced was so fragile that it would crash if you just looked at it wrong or the temperature in the room wasn't perfect.
While others with the same number of years of experience had massive systems in production for years with not a single bug reported by the happy users.
I know I got into such developement hell myself. Fix a bug here, results in braking something there. Experience surely helps in avoiding it .. but even senior devs can make a mess. Otherwise there wouldn't be so many projects canceled.
So sure, agents can multiply a mess in a amazingly short time, but .. that is up to the humans guiding them.