Over my guitar making career I’ve heard lots of ideas about guitar design and how to achieve great native sustain in an instrument. Sustain can be achieved electronically but my discussion is not about that. My thoughts on the matter are this:
When you pluck a string you introduce energy to the instrument. The instrument reacts to the introduction of energy and begins to respond by vibrating and absorbs the energy and gradually dissipates it. The time it takes for the guitar to stop resonating will demonstrate how alive the guitar is and how long it will sustain. How long it takes to ebb away depends on many things about how the instrument is made.
It makes sense to me that a lighter and very well constructed instrument will vibrate longer than a heavy one or an ill-constructed instrument. The physics says that it takes more energy to get a heavy guitar moving than a light one and if it is a hollow guitar with sound holes, it will move air and be naturally louder and have more dynamics and depth to the tone.
Some will argue and there is no right answer as to how to achieve the tone you’re looking for. I prefer to start with a naturally superior instrument and use less effects for greater results.