I don't get why people are viewing map simplicity so negatively

I have to call you out on this. It's patently false. NLC itself is almost entirely made up of reused assets - the tiles in the floor of NLC are from Mu Lung, in Krakian Jungle were from Shanghai, in Bigger Ben were from Kerning. The clockwork in the BG of Bigger Ben is recolored from Ludi, monkey statues from Floating market... Hell, the first two monsters you see are a recolored mushroom and a recolored slime, and that's not even getting into the randomly unchanged Firebombs and Jr. Yeti. The only original things in the first installment of NLC were the monsters, NPCs and the prominent buildings (Yetinx, Affluent Tower, etc.) Granted, that changed with later updates (Haunted House was quite original, and Crimsonwood didn't reuse anything to my knowledge, except possibly ropes.) There are a lot of things you can pull out on the new alien crap to show how bad it is, but that one isn't such a great choice.

Okay, you're right on that one. But many of them such as re-used tiles were.. well, almost unnoticeable, at least to me. Because they weren't out of place.

Here are some examples of re-used stuff.

 Object and ladder

 Object

Most obviously come from destroyed Leafre and, really, since there have never been trees like that in the area, they don't really make much sense. Others are re-used from the unwelcome visitor event we've had around 2 years ago.

One of the worst ones

 Spoiler

Aw.. come on.

Except for some little things such as unchanged Jr. Yetis and Firebombs, close to everything in old NLC was harmonic. Therefore re-used stuff mostly didn't catch your eye as much as now. On the other side - but it could just be me - many of the map objects they have used for this expansion (especially those in maps with mobs) really look out of place.

Also, as you said, at least they took the time to re-color some of the stuff, and there were several objects coming from areas which GMS had never seen before, so most of the players who didn't have a thorough knowledge of the game could consider them "new". Hell, even I didn't notice how some of those were re-used before you told me.

I don't know. The old Masteria just gave me a mysterious feeling that, with these plain maps and re-used stuff I now don't have anymore. Before I thought about Jr. Yetis and Firebombs inside the Bigger Ben as something mysterious which would've been, later on, explained by a quest of a future expansion, just like everything else. And it'd have probably been like that, knowing Prose. Now I just see re-used stuff as a sign of lack of imagination.

Maybe because it's actually like that. Afterall, it's not the first time we hear about aliens, is it?