I think it really varies, especially by genre.

Going back to the dark ages of gaming, some of the more fully-realised genres are quite playable (shoot-em-ups, 2D platformers) whereas some just lack the playability and QoL that the genre successors have (RPGs and, in particular, I'm thinking of the grandaddy of all RTSes: Dune - if I remember correctly, you can only issue commands to individual units one by one and not groups, the pathfinding is really lacklustre etc. which makes it borderline unplayable these days.)

There was also a major shift that occurred somewhere along the line in the gaming industry, I'm guessing it's around the late SNES/Mega Drive (Genesis) era as things moved towards PS1 and that generation of consoles; prior to this era there were a lot of ports for consoles and the arcade was king. What that meant was that a lot of games were designed to be short, with a high demand on skill and/or luck, and (at least for the time) thrilling. This means that a lot of games are extremely difficult by modern day standards because if you could get the player to chew through lives quickly playing a punishing game then you are also chewing through their quarters which makes the whole venture more profitable.

So a lot of the older style of console gaming was either a direct port of these unforgiving cash cow games or they mimicked the same style.

Obviously you get games that are outliers to this, such as Moonstone: A Hard Day's Knight or Wonderboy III: The Dragon's Trap which, despite the era, weren't designed to gobble up your loose change and you get the later entries in the Contra series such as Contra: Hard Corps (that seems to have been renamed to Probotector in some regions, possibly due to the Iran-Contra Scandal lol) which are still true to their arcade game roots and they really have the difficulty and demand on skill of a game like Ghouls 'n Ghosts. All of these games mentioned in this bit are very playable today despite the eras they were released in.

Nostalgia definitely plays a factor but I'd say that a lot of Game Boy games were an outlier to what I've described above as direct ports or near-direct ports weren't a possibility so the games had to be designed from the ground-up and often they were built around a non-coinsuck model.

There were games that were blockbusters and genre-defining in their time, some of which have held up and some of which absolutely have not, but if we remember or imagine back to the era that these games were released it was a content desert by today's standards which meant that a significant part of the allure of many games was that they were an antidote to boredom as much as they were fun to play.

It's no surprise that people of our generation typically don't play solitaire or mahjong whereas the older generations were and often still are avid fans of these games. To us they probably seem boring or maybe enjoyable for a round or two but prior to the advent of mobile phones and the internet, people spent a lot of time waiting or filling in time and so this goes a long way to explain why these games used to be so popular - because it was a good way to kill time.

It's a bit like if you took kids camping or whatever and they didn't have access to phones and tablets - I'd expect that playing tic tac toe in the dirt and similar sorts of games would dramatically increase in popularity.

I guess what I'm trying to say is that there's a lot of games that were good for their time or even today are good, but not great, that don't really deserve a second look unless you love the genre or the series or you're an aficcionado of retro games but there are some games that are really great even going way back into history and those are worth trying out; this is not dissimilar to movies from 50+ years ago.