comparing Fire Bolt to Booming Blade is like comparing Fire Bolt to Mage Hand.
I feel that's hyperbole.
While it wouldn't be a perfect comparison, it is not that difficult to compare and contrast different attack cantrips.
The DMG already informs us that 1d10 damage is a baseline for cantrips; that can be seen in Fire Bolt and Eldritch Blast. We then have Poison Spray, a cantrip where the designers indicated that a drastically reduced range is worth an upgrade to 1d12. Another cantrip, Acid Splash, indicates that the ability to hit adjacent targets is worth a drop to 1d6 damage. It is rough and dirty math, but one could say that combining both elements would result in a baseline damage of 1d8.
Greenflame blade, one of the SCAG cantrips and a cantrip meeting the description above, indeed has a 1d8 damage die. On the other hand, it has a number of additional benefits. Chief among them is the ability to do an additional ability modifier of damage to the initial target. Additionally, this cantrip stacks with everything else that would enhance a melee attack: magic weapons, divine strike, sneak attack, battle magic, magic spells, et. al.; other attack cantrips are rarely ever so lucky.
Those are some sweet, sweet bennies for a spell we'd expect to do 1d8 damage per tier.
I reckon you would probably have a higher DPR with TWF than with Booming blade.
The scenario you outline, using TWF as a way to increase your chances of getting a sneak attack, only provides a benefit in outcomes where the primary attack misses but the secondary attack hits. In any other outcome, ignoring the minor pip of damage from the off-hand weapon, it is no different from using cunning action in lieu of an off-hand attack.
That outcome only represents a small percentage of outcomes, meaning that extra chance at sneak attack damage only represents a small increase in average damage.
