So... How about you find that instruction that empties memory, reverse it so that memory is never emptied and...
Because then you'd have to save the game state, and as far as I know that isn't included in savegames etc. You would specifically need to write new code that would make exceptions for certain things - for instance, get rid of corpses, guns and so on but retain enemy deaths (probably requires assigning a unique variable to every enemy in the game, something that hasn't been done I'm sure), or even add a new timer that says "okay, 3 days until respawning enemies." In other words the game was never built with it in mind, so it's not something mods can fix without an SDK.
Also if you did exactly as you proposed, i.e. never emptying the memory... it would completely ruin the game's performance and could lead to data corruption very quickly. The entire point of memory management is to mark and unmark memory as used when necessary, and if you don't do that you tend to end up with major problems. And as I said, if you don't include that in a savegame anyway, it's all going to disappear the second the player quits the game (or, his/her computer crashes, which seems more likely in this case).
Besides, saving every single change to the game world gets really, really tough. The way games like Skyrim do it is they assign a byte to changes made to the default game package, rather than the entire game world, otherwise you'd have a massive save file. The performance problems with the PS3 version of the game were a direct result of save files getting too big and bloated for the console's small memory capacity to handle. And of course, even Skyrim resets stuff every few days to make sure save files don't get too big, though this kind of factors into gameplay as well (i.e. you can always go back to an old dungeon and replay it against new enemies if you want).
This sounds exactly what Ubisoft were looking to avoid with Far Cry 2, except they arguably went overboard or decided constant respawns were good for gameplay - in fact, if I recall correctly, that's exactly what they did do after playtesting found that players were too bored driving around without action every 2 minutes.