Most attacks are likely to hit. You're effectively increasing the chances of a crit by 50+ percentage points. Now, again, it's only really useful for attack roll spells but Chromatic Orb would act effectively as a spell of a much higher level. For example, it's basically making a 1st level chromatic orb a fourth level spell in all areas except counterspell.
Sure, but you are doing so
at the cost of an action. In that action you could have attacked instead.
2 attacks that hit are
better than 1 attack that crits in almost every situation, because only damage dice double. Two attacks that hit, one the round before, don't require concentration, and don't require pre-commiting to a target, is insanely better than 1 attack that crits.
It is a chromatic orb cast ...
that takes 2 rounds to cast. That is a really really sucky level 4 spell (actually it already was a sucky level 4 spell, now it is abysmal).
At level 1, you could orb (3d8) then fire bolt (1d10) for 19 damage. Or you could true strike concentrate for a round, then orb (6d8) for 27 damage. At a higher variance.
Suppose you have +5 to attack, and the enemy has 16 AC. So you hit 50% of the time for 13.5 with true strike, and 9.5 with the other one. But the other one has crits, while true-strike gets nothing; so now it is 10.45 vs 13.5.
Now suppose 1 in 5 times your concentration is broken, the enemy dies before you act, or the enemy gets behind cover. So now the true strike situation drops to 10.8, while for the fire bolt situation you can reaim your bolt at a new target.
And the orb + bolt situation could have dropped the enemy before it got another action, because its damage was front-loaded.
By level 5, the fire bolt damage goes up to 11, while the true strike combo remains the same, unless you burn higher level spell slots.
We now compare level 3 witch bolt (+7 to hit at 3d12 * 2) vs a true strike + level 3 orb (+7 to hit at 10d8) on an AC 16 target. They hit on an 9+; witch bolt deals 25.35 damage on average (assuming crits double all dice), while true strike + orb does 27 damage. A 6% edge, easily lost if you lose concentration, cover, or the creature dies.
At level 11 we can use a level 6 slot. Witch bolt does 6d12*2 or 78 damage over 2 rounds; true strike + orb does 72.
Add in advantage and true strike combo pulls ahead in damage per 2 rounds. Which as a cantrip being used optimally it sort of should? If it isn't ahead in damage in an ideal situation, shouldn't it not be chosen?