See again different assumptions.
I expect the mage bobbing up and down etc to get targeted by magic missiles or save damage effects - not by whatever the least favorable attack might be.
A lot - keeps coming back to this - a lot comes from how diverse the challenges are and how much of them are just the brute force more manageable options.
You keep wanting everybody to create forum polls - do you think the forum is in any way a representative sample of anything other than forum folks?
So, NPC spell casters who have magic missile are a dime a dozen at your table?
At our table, it's mostly monsters. Sure, there are times when NPCs cast spells, but Magic Missile is very easy for a wizard to avoid. Minor Illusion of a box around himself (can't target MM on a foe you cannot see). Shield spell. Be behind total cover. MM is actually quite lousy as a DM attack against a 5E wizard once it is done once unless the DM rules that the simultaneous damage of magic missile is separate sources of damage and requires 3 concentration rolls (which is a crappy ruling for death saving throws, but YMMV). If the DM throws MM, then always have Shield spell available to thwart it in the future. Meh. No way I would take WC just to prevent NPC arcane casters from MMing my PC.
As for least favorable attacks, more than 21 points of damage from a single source is REALLY REALLY rare at low levels. Almost all concentration rolls at nearly all tables are DC 10 below level 10. Typically, shy of a critical, a creature probably has to usually be CR 10 or so before they do more than 21 points with a single non-critical attack (there may be some rare exceptions). Spells like Fireball can easily do more than 21 if the save is failed, but at that point, the low level PC has taken an average of 28 hit points (shy of Absorb Elements) and concentration is the least of his worries. 4th level wizards typically have either 22 or 26 hit points, so … I'm not seeing where WC helps here. Regardless of circumstance, if the concentration roll is above DC 10, the 4th level wizard is already toast. At that point, even higher level wizards are in serious trouble.
Yes, a forum poll is a representation of forum folk. However, forum folk tend to be more knowledgeable on average about the game than non-forum players.
I think that your implication that at your table, encounters are challenging and at tables where players of wizards don't take WC at level 4, encounters are wimpier is greatly in error (or did you mean something other than wimpier when you said "more manageable"?).