This is again under the assumption that lightspeed ramming turns things to swiss cheese or whatever. But the way it’s depicted in the film it doesn’t seem like the lightspeed is necessarily the most important element to what would be massive destruction from the collision anyway. Why didn’t the Naboo fighters lightspeed kamikaze? Well why didn’t they just plain old kamikaze? Surely that’d cause some serious damage too.
Because lightspeed is depicted as massively more effective in TLJ than a normal ramming maneuver? Don’t forget that the debris from the Supremacy impact sliced up a bunch of Destroyers as well, something that is highly unlikely with a slower impact.
Based on the previous movies, an impact at sublight speeds might not do enough damage to destroy a single ship, let alone a dozen capital ships. In Rogue One, a transport impacts a Star Destroyer causing only minimal damage
https://www.youtube.com/watch?v=Z4GppY9Vh0k
…but in ESB, a somewhat larger asteroid traveling at a similar speed is able to take out a Star Destroyer’s bridge:
https://www.youtube.com/watch?v=4gh7AcuxfxI
A ship, even one as large as the Raddus, probably wouldn’t be enough to bisect the Supremacy without going at relativistic speeds. This goes against canon, where the ‘acceleration’ is thought to be an illusion.
http://starwars.wikia.com/wiki/Pseudomotion/Legends
Is it possible that a ship entering Hyperspace first accelerates to ‘88MPH’ or something? Sure, anything’s possible in this made-up universe. But this just complicates a straightforward conceit of Space Opera storytelling by its military implications, and I think it’s frankly irresponsible to make this change to the rules of the universe for one (albeit cool) moment.