'Reach's engine is 90% hacks made by Bungie to make systems originally implemented in Halo 1 and 2 do things they were never meant to do'
lol. the only systems left from halo 1 and 2 would still be doing the job they needed to do. Reach's engine has a completely new LOD system, doesn't have Halo 1's physics, has a better lighting engine than Halo 2, can handle more draw distance than Halo 3, has an entirely new multiplayer gametype engine.. the tag system didn't need any total rewrites to keep working.
Shit, Halo 1's engine didn't even support bumpmapping bipeds. They could only have bumpmaps on world geometry.
"completely outsourced multiplayer to CA"
We know CA did Dominion and some maps but we also know for a fact CA didn't do the entire thing. CA also released two games while Halo 4 was in development. Don't think they would have had the time if they were entirely co-devving the game.
paraphrasing "once we had Slayer running, we kept running out of script memory"
Uh, the only game script that constantly ran out of memory in Reach was Invasion. Slayer is one of the smallest scripts you can make, even WITH GO/PO hooks implemented. If you think about it, Flag Slayer is Slayer with the entire CTF engine running on top of it.. and that's with Neutral Flag support on top of that, and the 'flaming feet' functionality on top of that. And they ran out of memory after implementing Slayer? Not everything in multiplayer is handled by Megalo, but let's also remember they cut the entire ability to play Elites in multiplayer, which would have given them loads more headroom. They even have the spare memory left -post ship- to add even more weapon skins, which are essentially their own weapons, on top of the existing ones. And more armor. Doesn't sound like 343 is under a memory crunch in the slightest.