>It's blindingly obvious, not a clever deduction, nor was this some sort of special circumstance (no slingshot around a black hole or some other very unusual thing, for instance).

It makes sense in universe. My research tells me it was mentioned in the EU. It just happens that none of the writers considered it, or employed it in a movie.

>Why won't this be the solution to every battle in the future?

I can think of a few reasons, most of which are the reasons kamikaze attacks aren't the solution to every battle in the real world:

1) Ships are more expensive than torpedos. I see no guarantee in TLJ that the maneuver doesn't pose a risk to the rammer, or that the likelihood more often than not would even be success. What we see depicted appears to be a desperation tactic, not something that makes sense as a primary battle tactic. It may simply be the case that the maneuver requires a degree of precision and luck that makes it infeasible in most circumstances.

2) The maneuver can be detected. It's clearly shown that the Supremacy had time to see the attack coming and could have prevented it, but they didn't. Part of the reason it worked is Hux's arrogance. If the maneuver requires proximity to pull off then it's likely that it leaves you vulnerable to attack if your opponent isn't a moron. Although this is the Star Wars universe, and Star Wars villains always seem to be carrying the idiot ball.

3) Tactics can be adapted to. If kamikaze tactics become commonplace, then ships will counter by spreading themselves out so as to minimize damage from shrapnel (which they should do anyway,) build smaller ships so as to be more difficult to target, mine hyperspace, something.

Why does the United States bother with an army and a navy if it can simply nuke anyone it likes from orbit? Why didn't kamikaze tactics allow the Japanese to win World War 2?

>Why bother with capital ships if they're so vulnerable to this simple attack?

Does it not make sense that they would be vulnerable? Would you have preferred that the Raddus rammed the Supremacy without going to lightspeed first? Should it have just bounced off in that case? Given the size and power these ships reasonably have to begin with, it's not absurd to assume that ramming under normal circumstances would be devastating, even without hyperdrives. Hyperdrives automatically make them exponentially more powerful, however, and that's not unreasonable.

>Is the Death Star even necessary if ships can be this powerful on their own?

The Death Star was destroyed by a farmboy with almost no training in a space fighter using two torpedos and telekinesis. That's far more ludicrous than Holdo's maneuver, yet no one complains about it.

>Why did the Rebellion wait so long to deploy this?
>Why doesn't the Empire, sorry, "First Order" use this on the rebels, sorry, Legitimate Representatives of the Galactic Government Who Are Somehow Still The Rebels?

Because this is a fictional universe in which events unfold for the sake of drama, and the writers aren't bound to a maximalist interpretation of the lore. The entire Death Star sequence was ripped from a World War 2 movie, after all. None of the tactics in the series make sense to begin with. Manned ships fighting in close quarters like naval vessels or fighter planes don't make sense in space, at all. But it's odd that a tactic grounded in realism is the one that so many fans consider the straw that breaks the back of the settings' credibility, and not, say, everything to do with Starkiller Base in The Force Awakens.