This question opens up a giant can of worms and potentially changes the entire paradigm of ship-to-ship combat in the Star Wars universe.
First issue: hyperspace is supposed to be an alternate dimension. Even in the 70's, we knew that accelerating an object to the speed of light isn't actually possible (I think). Except now, it look like hyperspace might be achieved by actually accelerating the ship to light speed. Makes for some serious weirdness.
Second issue: Admiral Holdo may have just invented the greatest anti-capital ship weapon of all time. Like, you said, you could strap hyperdrive's onto rocks and suddenly you've got massive ship killers. Or (and this is a question that's always bothered me about star wars) you could build a missile, maybe about half the size of an A-wing fighter with a hyperdrive in it (I think A wings have hyperdrives in one of the movies) and just program them to seek and destroy enemy ships. If it was moving at the speed of light, it would obliterate pretty much anything it came in contact with (In fact, if Holdo's ship was moving at light speed, the incredible part is that snokes entire ship wasn't obliterated (relevant xkcd). This would be THE new weapon. At the range where Holdo launched her assault, the laser batteries (and calling them lasers does hurt me a little bit) on Snoke's flagship weren't powerful enough to penetrate her cruiser's shields. If she had a battery of these missles should could simply have launched them from standoff range. The missiles could be shot down while deploying (launching a missile into hyperspace while it's still on your ship just doesn't sound smart), but the most powerful weapons on what is probably the most powerful ship in the most powerful military force in the galaxy would actually be powerless to do anything else. And considering how bad laser batteries in star wars are at shooting down fighters, I wouldn't be too worried about point defense.
Some potential issues (and solutions!) with this hyperspace missile:
If hyperspace isn't actually light speed travel, but massive acceleration is merely part of the process of reaching light speed, then were would be a relatively small window in which the the missile could impact before being in a different dimension. Let's assume this is true, because otherwise starship combat becomes suicidal.
This makes the missile slightly less unstoppable, but there's no reason that the missile couldn't evade (you know, like...droid starfighters). Or maybe you launch missiles in saturation waves, or you have dedicated bombers that carry these hyperspace missiles (because seriously, why the hell do SPACE BOMBERS have GRAVITY BOMBS).
Presumably, the creation of such a weapon would result in some better point defense (point defense in Star Wars is pretty pathetic). Assuming that the missile have to move at starfighter speed to "detonation" range, i.e. launching into hyperspace, defense would probably take the form of escort fighters (which already exist) and missiles launched from starships (and why the hell do missiles only come from fighters in star wars. Nobody thought to make a giant missile and put it on a capital ship? WHERE ARE MY PHOTON TORPEDOES.)
Main point? The age of starship battles is dead. Just like in the real world. We'd have escort ships and starfighter carriers because missiles defended by fighters would be more deadly than mysterious energy packet launching guns (seriously, I think the "lasers" that Snoke's flagship was carrying had projectile drop. LASERS. WITH PROJECTILE DROP. IN MICROGRAVITY), and the best defense against them would be fighters screening the capital ships outside of "detonation" range.
So Star Wars has some weirdness in it. Always has. Probably always will.
I still had fun.