I'm a few years older and confused by this as well, considering back during those times was seen as the first golden age of gaming. 

The thing is, every single genre will always have their stinkers and their winners. You've just got to:

I listed a few that are still timeless and enjoyed today (Castlevania/Megaman/Metal Slug are prime examples of winners). You are probably better off looking at a few LP's on Youtube to get a look and feel for some of these games, because not all games within any given genre have greater depth (some like R-Type are literally just mission based and that's it). 

Take for example two winners: C&C Red Alert 1-3 vs Age of Empires 1-3

C&C Red Alert is a spinoff of "what if" C&C games with zany cutscenes and units, but their stories are pretty shallow and they often rely on the zaniness to carry the stories they tell.

Age of Empires on the other hand has more depth with it's realistic historical texts and factions based on irl historical armies. 

Both games franchises are great and fun, but both are also different, in that one is whacky and amped with zaniness, whilst the other is trying to tell you an in depth story with more tactical throughput.

Another example could be League of Legends vs Heroes of the storm. Both tell barely any stories, but League has greater tactical depth with it's massive hero roster and item shop, whilst HotS has a simplistic style of gameplay for a Moba, but comes with a cast of characters from existing Blizzard IP's that I know well enough. 

I chose HotS because it was simplistic and didn't rely on me having to adopt meta strategies to have fun. Yeah it's a bit more shallow than LoL, but it's still fun, and that's the point, some are fun and some have more depth (which can still be fun for others).

Yeah, but it's also the direction through space, you have two options, move left or right and sometimes up. When it comes to a boss it just seems so... basic. Like what can the developers do to creat a big differential between bosses. Oh, I gotta jump three times now to avoid this attack, that two whole more Jumps, like wow. 

Dust An Elysian tail is one of my favourite games ever BTW this is a new realisation to me. I suppose I went on too much of a soulsborne binge lately and now 2d just seems overly simplified in my mind and easy to my fingers. I suppose after Nioh 2 every games is gonna seem piss easy. 

Dust was a nod to Vanillaware games. So try those out and like I said before Gaurdian Heroes.

I gots Odin Sphere installed. Pretty excited to play it actually with how it's been described above and the short video I seen of it. It certainly does look a visual treat with all the effects, if nothing else. Oh and Overlord Univorn, you don't play the game, you set up the fights and watch them play out... I mean if Vanillaware can make that work they are truly wizards. They did get me addicted to a 30 hour long visual novel in 13 Sentinels, they are a special dev for sure. 

But that's the thing, it doesn't need to have added artificial depth for the sake of adding depth. Some genres just work with simplicity and some don't, but side-scrollers are what they say they are, a game where you scroll from left to right, up and down, and it's a formula that has worked well for decades. 

Some of them will feature back tracking (Castlevania, sometimes Megaman, etc), which can add a form of depth. There are some bosses that come in stages where you have to avoid certain attacks or transverse moving terrain, and that becomes it's own form of depth. With side scrollers, you won't really get 3D levels of depth because then it wouldn't really be a side scroller if it had to abandon one of it's core principles. 

Don't forget, there are also plenty of 3D games where combat is made super simple (and there's honestly nothing wrong with that, not every game has to be hard or a souls-like), and some that aren't, but it's how you go about adding in those systems and mechanics to benefit the player or to compliment the game's flavour. 

Dust is also one of my personal faves, along with Guns Gore & cannelloni, and both of those aren't really that deep, but they each have their own types of flavours and mechanics that add a bit of spice to make them interesting and which allows them to stand out from the crowd. 

See for me personally, as I've grown older, I still find myself choosing "normal" mode, and never really stray further than that. I play games mainly as a form of entertainment. The only time you'll see me wanting to challenge my brain is when I'm playing RTS, because I like to run "calculation simulations", which to me is akin to white noise (I can play most RTS that aren't Starcraft with half my brain focused on something else while just issuing orders/building most of the time).

I think if you're really into challenging yourself, you should look around for games like Super Meat boy, or just try playing some side scrollers than contain perma death/extra hard modes to ante up the stakes. personally I'd just play them a few times over, then call it a day (much like I do for open world and RPG games these days). 

You make a great point and you're probably right. I suppose it's an adjusent issue and ya need to be in the heads space for certain games. Guns gore and Canoili was pretty dope. 

Oh and fuck Super Meat boy, that game is something I'm sure they make you play in hell. The pain that is infused into the core of that game is not found in many others. 

Just done the the first level in Odin Sphere and I'm already super impressed, it's got combos, it goes extra with directional movement and more than just left right with movement mechanics as well as juggling enemies and the combos feed into movement as well. Feels like I'm playing DmC after a few hours of buying combos and I'm not noticing that I'm stuck to a 2d plane. This is what I'm talking about, goes hard right out the gate. Disregard my previous comments on 2d games, they were misinformed by five mediocre games in a row.

BTW, cheers to everyone who suggested Odin Sphere, I can see this is gonna be a treat. Kisses.

Aye.

It's like you said, GG & C was a dope game, but I tried Super Meat Boy and I was put off after dying time and time again, so I learned that I liked games like the former and Dust more than the ones that challenge you to the bone.

There are some games out there that do want to genuinely tell you a story, and then there's some that are just there to act as fluff, and then those that are just pure challenge. You just have to try sifting through the sands of each genre to pick out the ones that resonate with you vs those that do not (like how I do with RTS, since I vastly prefer C&C/AoE style RTS over tactical RTS like Commandos/Total War).