Let's take your example of someone who stops purchasing anything. Without any clarification by the customer, the company "could think that the person just doesn't want to purchase as many things, or can't purchase as many things currently due to either lack of funds or issues with life," same as you allege with someone who reduced purchases.
This is false. No purchases shows dissatisfaction. Some purchases can mean anything. No matter how many times you try to justify your purchase of games from GoG while engaging in a "boycott", it will never mean that your "boycott" isn't anything but a lie.
This is wasted energy since you will are unlikely to admit to tripping yourself up, but for the benefit of other readers and participants...
Say there is no boycott topic at all, but there are still customers who have stopped purchasing entirely as well as other customers who have drastically reduced purchasing. Your assertion that "no purchases shows dissatisfaction" is completely unprovable and indistinguishable from someone else claiming "reduced purchases shows dissatisfaction," which in a vacuum (i.e. without the context of a thread like this) is also unprovable as indeed either customer may be perfectly satisfied with the store but unable to purchase for financial reasons, other life issues, etc, as you suggest. This is why a topic like this is valuable as it provides evidence to prove there actually does exist dissatisfaction.
Consider that with both types of customer, but without a voicing of their concerns, GOG would only be able to guess as to why either customer has changed their buying habits. For instance, maybe someone who stopped purchasing entirely simply has acquired all the games they want, or as you suggested could perhaps have lack of funds/new real-life concerns in the present now despite not it being that way in the past. Notice that the exact same is true for a customer who drastically reduced their spending? That is, maybe the person who drastically reduced their spending acquired most of the games they want, or have issues with funds/real-life etc compared to the past.
With the boycott topic, GOG can directly see in most instances why a customer has reduced or ceased purchasing, since the customers are able to directly voice their reasons.