Diamond Rats, that term covers a few different types. NPC Mining Fleets are Diamond Rats which consist of either Empire or Pirate NPC’s. Having those in High Sec systems presents both a problem and a challenge.
Problem is they can easily mine out all Asteroid Belts in a system within a few hours. Challenge is surviving or defeating their Attack Battle Fleets which spawn when attacked or due to bad Faction standings.
In my opinion the main issue that breaks immersion is having Pirate Mining Fleets active in High Sec systems. Same with the Pirate Forward Operating Base’s in High Sec systems. Those should be in Low Sec systems since Pirate NPC’s are located in Null Sec space.
Pirate FOB’s, Triglavian Invasions, Sleepers, Roaming Triglavian / Drifter Gangs, etc should not be in High Sec space, that is high level content which is suited for Low, Null and w-hole space. That advanced content is being forced on High Sec space in an attempt to force players to leave High Sec, all because a couple of Dev’s decided the game should be played a certain way.
Disregarding the fact that the content breaks immersion, after adding that crap to High Sec space, CCP then removed low to mid level combat sites, not to mention implementing their hairbrained Scarcity and Ore Redistribution crap. Basically CCP will implement whatever they can in an attempt to force players out of High Sec space.
The question they should be asking is why do majority of players choose to remain in High Sec space?