Phrase for a useful or comforting illusion

aphorism

a short clever saying that is intended to express a general truth

-Cambridge online

Additional to Cascabel’s answer:

Proverb a brief popular epigram.

Adage a saying, often in metaphorical form, that typically embodies a common observation.

Saw - Maxim or proverb.

Etc....

All of these convey a sense of “universal truth” but are often at their core little more than hot air.

For example: You break up with a significant other and someone tells you:

It’s better to have loved and lost than never to have loved at all.

On the surface, this sounds like a universally true statement. Yet, as it does nothing to salve the pain you’re feeling, its “universal truth” is suspect.

The term is Common Knowledge

It sounds like you are looking for the common touch to the idea, not the erudite of Cascabel’s or David's answer.

That which is "widely held, yet seldom scrutinized" is Common Knowledge. Being common it is taken for granted and requires no examination. As knowledge is it what "everyone knows" and so is widely held though seldom grasped. A great societal force pulls along and comforts people to nod and agree rather than spoil the trip with a pointed question.

More useful is common sense. Likely because it is so rare.

A security blanket, as used by Linus in the Peanuts comic strip. All of the Peanuts characters are “positive”, so the term is more whimsical than hurtful.

https://peanuts.fandom.com/wiki/Linus%27_security_blanket

This term could be used in a context like the following:

“Let’s start out with X as a kind of security blanket...”.