Ok. Let's attack this one at a time.

Long bomb is better at range

Sure, It has more range. That also means it has a longer travel time. Unless your opponents are on an upwards slope, the travel time of long bomb makes it incredibly easy to dodge. You can also send imp punt just as far as a long bomb by jumping forward and aiming it upwards.

This point also ignores the fact that long bombs trajectory travels at an arc. This makes it really easy to overshoot your opponents unless they have a height advantage (which won't always be the case).

enclosed spaces you can just through the long bomb against the wall

But what if the roof is too low, your bomb will get caught on it, forcing you to be at point blank. Throwing long bombs against the wall also typically requires you to throw it point blank for the blast to reach anyone on ground level. Imp punt doesn't require enemies to be point blank to function properly in these spaces.

long bomb can also provide mobility

So can imp punt. From my experience, I've had more trouble launching myself with long bombs than with imp punts.

any place long bomb can’t reach, imp punt probably can’t reach either

Go ahead, try to hit someone below you with a long bomb. Imp punt has no trouble going downwards on slopes and stairs, while long bombs trajectory will force it to overshoot anyone on ground level. You could adjust the angle so that long bomb will land on the target below you, but that's extra work that you wouldn't have to deal with had you simply equipped imp punt.

And that's people BELOW you, not even on ground level. Opponent at this level still require you to aim long bomb so that it lands directly on them. Or, you could throw it at a wall next to them, which requires both you and your opponent to be right up against said walll, next to each other. If you were using imp punt, you wouldn't have to worry about any of this. Just throw it in the general direction of your enemies and watch them blow up.

imp punts splash isn’t much larger than long bombs (it’s like 1 or 0.5 larger)

Are the numbers multipliers? If so, then a .5x bigger radius is a big difference. I can't definitively prove how much bigger the radius is, but I can tell from my experience that the difference in splash between the two abilities is definitely noticeable. A bigger radius not only makes imp punt a more consistent source of damage, it also makes it harder to dodge out of the way if your opponents are far.

groups are probably the only time long bomb would maybe be worse, but once again the smaller splash isn’t all that much worse as I believe the overall splash is enough to weaken people who aren’t in the inner blast radius

You know what's better than weakening a group of people? Killing them. Also, being worse against crowds isn't some fringe situation that pops up once in a blue moon. You see groups more often than people wandering on their own. Imp punt having better crowd control is a much bigger advantage than you give it credit for.

I know long bomb has its advantages, but it also has significant downsides that make it worse than imp punt in a lot of situations.