"Everything works.....for about 6 weeks."
Training Age
As you probably know, this has to how long you have been training, not your chronological age.
The General Adaptation Syndrome
Hans Selye, MD, PhD, determined this around 1923.
It had to do with individual adapting to diseases. If they didn't adapt, they died. It occur with all living things.
Covid is a great example. Covid is now adapting (mutating) so that it survives.
Periodization Training
The foundation of Periodization Training is The General Adaptation Syndrome.
As you stated, "Everything work but nothing works forever".
Thus, at some point the body will adapted to any new training program. Once the occurs, progress stops (metaphorically progress dies).
When progress stops, a new Periodization Training Program need to be implemented.
Novice Lifters
Someone new to training adapts to training slowly. That means they can preform the same training program longer and continue to progress. For Novice LIfter's a 6 - 8 week Training Cycle usually works before they need to change it.
Advanced Lifters
Someone who has been training for a while adapts quickly. This means they need to change their training program about every 4 - 6 weeks.
Diet Periodizations
The General Adaptation Syndrome applies to diet (loading or gaining weight), as well..
The MATADOR Weight Loss Diet
This weight loss diet is based on decreasing calories for two week, decreasing them for two week, then repeating.
Research determined that it take the body approximately two week to adapted to changes in calorie intake.
As with training, once adaptation occurs, weight loss stops. That because you metabolism decreases to your new calorie level.
The research found after two weeks, increasing calorie intake. Doing so, increases you're metabolic rate.
After the two week calorie increase, you then drop you calorie intake. Since you metabolic rate has increased, you then lose weight/body fat because you are burning more calories than you are taking it.
This approach is a "One step back, two step forward approach".
Gaining Weight
The same protocol is applied for gaining weight/increasing muscle mass.
However, you increase calorie intake for two weeks, then decrease calories back down to what is was when you were maintaining your body weight.
Bodybuilders Bulk and Cut
The MATADOR Diet approach is the same Bulk and Cut that Bodybuilder have used for decades.
However, one of the issues with many Bodybuilders was the drastic increase in calories and then decrease in calories.
Independent Research On Calorie Intake
Research two individual, independent of each other, came to the same conclusion.
1I Greater muscle mass was maintained and more body fat was lost with a 20% decrease in calorie intake.
2) Muscle mass was increase to a greater degree and body fat mass gain was minimized with a 20% increase in calorie intake.
General Rule
Once progress stop with a Training or Diet Program, something need to change.