cycles become less productive over time due largely to saturated steroid receptors in the body. how quickly you reach saturation is largely genetic, and differs with compound. i'd give up 50lbs to have virgin receptors again.
people are telling you that pct is key for good reason. losses post cycle happen due to test levels crashing when exogenous test is discontinued, allowing cortisol (catabolic hormone) to become dominant. with proper pct, you can restore natural hormonal production before all the exogenous hormone is exhausted.
it's not an absolute that you lose x amount of muscle after each cycle. you can keep your gains. you simply can't expect to see the same gains from the same cycles, year after year. the longer you use steroids, the less effective they become. . . and the more creative you have to become in order to grow.
these are the reasons we don't see 500lb monsters around every corner (not muscular attrition thru post cycle catabolism). the really big boys are genetic freaks, and by that i mean, they have less shbg, more capacity for free test, and more receptors than 99.9% of people on the planet.
all steroids suppress hpta to varying degrees, so test needs to be part of every cycle to replace what is suppressed naturally.