Whilst I agree with your numbers points, the idea that lightning is the visual and thunder is the sound is pretty fantastically silly in the context of the spell Lightning Bolt. There is absolutely no possible way, if we agree that Fireball makes a noise (probably an enormous boom or woosh or both), that an electrical discharge, however magical, of the size involved in Lightning Bolt isn't going to be pretty ear-splitting - at the least equally loud.It doesn't get the fame of fireball, but it still does damage above the recommended amount based upon guidelines, and when it is useful, it is incredibly useful. It is resisted less than fireball, and it can impact enemies spread over a much greater range (they can be 100 feet apart rather than 40 feet apart). Also, and often overlooked - it is lightning, not thunder, so it is much quieter. A fireball announces your presence to those outside line of sight - but a lightning bolt may not (DM discretion, of course - but fireballs make a low roar, while lightning bolts are not described as making any sound, and we all know that lightning is the visual and thunder (which is a separate thing in D&D) is the sound).
While it's hard to get a bunch of targets perfectly lined up for a lightning bolt, it's far easier to use it in the first place than fireball. I play a Light Domain cleric, and last session when I cast fireball, I got a response from one of the players "Whoa, you can fireball?!". Yes, Mike, I can fireball, but since I care about the well-being of my party and am not an evoker with Sculpt Spells, the fireball doesn't see much use.Being a line just makes it impractical (and that goes for other line effects too). It's a red-letter day when you can hit three opponents with a line effect, where radius effects can affect multiple targets much, much easier.
Sure if you have a lightning bolt prepared you can use it to pump 8d6 damage into a large single enemy with not too many problems. But by the time you're 5th-8th level, large single enemies won't be too frightened by that, and the archer ranger with hunter's mark and colossus slayer will be doing almost as much every round, at a longer range and not limited by spell slots.