@lepomis Your body is a machine at adaption and will get used to a lot of things - we call it plateauing. This can happen to certain exercises, or modalities. For example, you've been doing Hammer curls for a year but your arms have stopped growing as much. So you change the exercise to zottman curls and suddebly you get more growth. Or switching from back squats to front squats.
As you do your training, your body will increasingly get resistant to the volume too. For example, you're training sets of 15-20, and growth slows or stops.
So you switch to sets of 5-10, maybe even 4-6. The heavy work will maintain your muscle, but no longer cause that higher volume stimulation, so that sort of settles down, and you gain strength. After a few months having gained strength, you'll become more sensitive to hypertrophic growth again, and switching the volume back may kick-start more growth, as your body has sort of 'rested' like I said, you body will get resistant to stimulus at some point. If you constantly training for strength, your adaption may also falter. The same can be applied.
However one caveat is that everyone responds differently - for a new gym goer this may take a year. For an advanced lifter it may be a month. Or for some it may not even occur, so the bottom line of 'if it works for you, keep doing it' still applies.
And yes it would be, but while there is a strong crossover between strength and hypertrophy, there is some conflict. High volume hypertrophy does not gain much outright strength. Low volume Strength may not trigger much muscle growth - both will provide some but this approach violates the first principle of training: specificity. Are you training to get stronger or bigger? Combining the suboptimal method of training one for the other with the adaption I mentioned above, one would run a hypertrophy phase, then a strength phase, not both at the same time. High volume creates higher levels of fatigue, which may conflict with your strength.
My problem with 5/3/1 is that if training for strength, the volume is quite low. The set of 5 is effective. Set of 3 is on the cusp, and will get you stronger but it's jumping ahead and fatigue will rise quicker.
My preference would be a phase of hypertrophy then a phase of basic strength. Then, as a competition or test day nears, a tapered approach of triples, doubles and ultimately singles. Again, however, if it works for you, then great, and these programs do work for some, it's just it's an approach I'm not a fan of.