Newer games are designed for you to win whereas older games, especially arcade titles, were designed for you to lose.
It's because especially in the current era, casualizing a game gives more sales, Making satisfying and easy to play game to make them accesible to allot of people it's where the money is at. That's not to say that difficult modern games don't exist, but there is way less of a focus on that.
I've seen far to many people being spoiled by modern day QOL and see punishment as a point of critique, wich is superbly stupid imo. Majority likes feel good games and they want convenience. Mastery and punishment are bad things for most modern day players.
If where talking especially the indi sector you could argue that some games on the hardest difficulty settings are harder than those ultra hard nes games. so not all is lost the options are there, and some games give a bigger challenge than back in the day if you want to punish yourselves.
I will say I do disagree that games are longer than back than, 2d platformers especially modern ones have roughly the same content compared to older classics, 3d games are rougly the same amount of time aswell with the exceptions being the huge open world games wich weren't that much of a thing back than again morrow wind in the OG xbox era exists with hundreds of hours of gameplay
Look at sunshine, galaxy and 64. odyssey ain't that much more content.
Or look at GTA V compared to san andreas, a ps2 game that has more content unless you go multiplayer in GTA V.
2d games might be shorter, but even modern 2d games have pretty much the same amount of content, these games never where supposed to be that long. it's a genre that you really can't compare to 3d games. 2d games are supposed to be hard in wich you need to learn a pattern to progress.
If you make a 2d game to long, lvl designs might end up boring. aka the reason why even modern 2d platform games have the same time span. 2d games will never have the hundreds of hours experience would make no sense, let alone that especially back than pretty much no one would be able to beat it than.
so in conclusion 2d games in modern era roughly the same size as quality titles from back than.
3d era games from ps2 gamecube era onwards again roughly the same time depending on the genre
The only main difference in time are the new genre's that didn't exist back than. that's where the differences are at, but even in ps1 era hundred hour games exist.
action games will always end up short compared to rpg's especially in 2d era, but in reality, rpg's usually use simpler grapics, less action less movement so even if it has more content, it's more simplistic and spread out compared to the action packed experience of say megaman x. While in reality in terms of dev time that shorter 2d game took more time to make rpg's usually have way less details compared to those fast paced action games and for good reason.
These games would never have dozens of hours of new lvl's unless they literally copy pasted stuff and that's not a great idea. let alone the memory needed, action based games used a hell lot more memory for less game compared to rpg's