Cost
This is not without cost to the warlock; he has chosen to use one of his two invocations to get this thereby forgoing other choices. In addition, he uses an action to cast it and an action to move it; unless your battles are very static he would need to move it a lot. Remember, the most limited resource any creature has is not its spell slots or hit dice; it is its actions, it only gets one per turn. Good players know this and they should be thinking every turn "Is this the best thing I can do with my action right now?"
Innovation
This is a very clever and imaginative use of the spell - this is something that you should encourage in your players; not discourage. I have had a wizard use Silent Image to create an picture of a hallway that the rogue could walk behind; this not only gave advantage, it also allowed sneak attack against, coincidently, a bugbear.
Disadvantages
A 15 foot cube of fog rolling towards the bugbear is going to negate surprise (its just not natural) and allow it to make an active perception check to find out where the PCs are in the cloud. The bugbear can then use its action to attack (with disadvantage); following which it can see through the cloud because "Physical interaction with the image reveals it to be an illusion" - sticking your morning star into it qualifies as "physical interaction".
It doesn't work that way, anyhow
You say: " because the party knows it is an illusion, they are never hindered by it". Where does it say that in the rules?
The relevant text of the spell is:
Physical interaction with the image reveals it to be an
illusion, because things can pass through it. A creature
that uses its action to examine the image can determine
that it is an illusion with a successful Intelligence
(Investigation) check against your spell save DC. If a
creature discerns the illusion for what it is, the creature
can see through the image.
There are only 2 ways to see through the image, "Physical interaction" or "use your action" and make a save. Knowing that it is an illusion doesn't allow you to see through it.
The image appears and moves naturally, otherwise it wouldn't be an illusion at all.
Pertinent text:
You can use your action to cause the image to move to any spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking.
Another example of this would be creating the image of a spectral, headless horseman charging your target in eerie silence.
When you use your action to move it, the image moves as it would do so if it were natural, maintaining the illusion. After all, it would be incredibly easy to discern something was an illusion if it just disappeared and then reappeared every 5 feet. Since this spell requires an investigation check to discern if this is an illusion, the image by default has to appear real.
A static, non-moving fire does not appear real, and thereby would not be an illusion. So any interpretation that this is still image doesn't make any sense, and doesn't address the spell at all. At no point does the spell state that the image is static.
Best Answer
You are correct that Silent Image can create a wall. Both spells belong to the Illusion school, with the "figment" subtype, and allow a will save to disbelieve the illusion. But each spell has its advantages and limitations.
Silent Image is a temporary, cheap, and flexible illusion.
Consumes a level 1 spell slot.
On the bard, magus, and wizard/sorcerer spell lists.
Range is Long (400 ft. + 40 ft./level).
The maximum size scales with the caster's caster level ("four 10-ft. cubes + one 10-ft. cube/level").
Requires concentration to maintain. A caster could have only one Silent Image active at a time.
If someone succeeds the will save to disbelieve the image, then they can partially see through it. According to the rules on illusion spells, it will appear translucent upon a successful will save, because the Silent Image is a figment.
Illusory Wall is a permanent and effective illusion of a wall or similar surface.
Consumes a level 4 spell slot.
On the wizard/sorcerer spell list only.
Range is Close (25 ft. + 5 ft./2 levels).
Illusion size is constant (1 ft. by 10 ft. by 10 ft).
It has permanent duration, where as a Silent Image requires concentration to maintain. With enough castings of Illusory Wall, you could eventually create a permanent illusory castle!
Unlike other figments, even if a creature succeeds the will save, the illusion is still effective. Someone may know the wall is an illusion, but they still cannot see what's on the other side!