While we don't know it is true in this specific case, there is, IIRC, quite a bit phycological research that suggest that it is generally true for modern humans.
It is not exclusively people who have a negative opinion who speak up. It is people who have
strong opinions. In both directions.
The vast majority of people don't have strong opinions. That doesn't mean they necessarily think good of things. Their opinions just aren't strong enough to speak out. Consider, for example, the many, many,
many people out there who stick with a job they hate, or a broken and unhealthy relationship, or various other things, not because they
want to, but because it's contrasted against uncertainty and possible loss.
"Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed."
And, to be clear, this doesn't mean their opinions are
bad either. They're just not strong enough to speak out either way. It is patently obvious that 5e was designed by only one slice of the existing community when it was called "D&D Next." That slice is now VASTLY overwhelmed by the new blood in the hobby. It is foolishness itself to presume that that new body is perfectly represented by the original sample set. New data should be collected--and not just the terrible surveys WotC has been running. Actually
representative and effective survey design.