I feel the same way to some extent. This potion has so much potential. It could increase any or all of the parameters in the game that involve chance. I know that Mojang doesn't want it to directly overlap with other chance-based enchantments, such as Looting, Unbreaking, ect. which makes sense, but there are still dozens of mechanics that a luck potion could reasonably apply to. Just to name a few of its possible effects:

• increase the willingness of villagers to breed

• increase the likelihood for iron golems to spawn--conversely, decrease the likelihood of hostile mobs to spawn

• increase the player's chance of critically striking with a bow, or disabling a shield with an axe, or not getting their shield disabled if they're the player whose shield is being attacked with an axe

• increase the weight of rare enchantments--i.e. the probability that a rare enchantment such as Infinity will be selected over a more mundane enchantment of equal enchantment level such Power IV

• increase the average yield of crops without actually raising the cap (e.g. harvesting potatoes increases average yield by .5 while still keeping the max yield at 4)

• decrease the accuracy of mobs that shoot at the player, such as skeletons and blazes (i.e. make them more likely to "miss")

• decrease the chance of fire spreading to a adjacent blocks

• decrease the likelihood of mobs to do things that are harmful (for instance, endermen picking up blocks, or hostile mobs to travel in your general direction whilst in their idle wandering state)

• increase the likelihood that bonemeal affects single-stage plants (e.g. trees), or the strength with which it affects multi-stage plants (e.g. wheat)

I'm hoping that they do quite a bit more more with this potion than what they have it doing right now. You're right that it feels altogether underwhelming at the moment.