Only thing I have against modern game 100%s are the collectibles. That's never been fun, not even back when GTA3 was doing it (but at least GTA3 gave you decent rewards to finding them).
Yakuza has a shit ton of stuff to do in its 100%, but I love it because I genuinely enjoy learning all the mini games. Less so with say... RDR2 with some of its requirements (again, mainly tied to collectibles) but that never stopped me seeing the ending to the game. So... I just ignored that stuff. Honestly, that's all I do these days. If I don't wanna 100% it I don't. I don't feel compelled to do it, if the game doesnt entice me I don't bother.
Also, I somewhat disagree that older games were more possible to 100%. You have games like Contra 3 or the Ghosts n Goblins series and I see people routinely complain about games like that on here because of how hard they are. And those just require you to finish the game (in GnG's case, twice, but that's not actually hard if you spend an hour or two actually bothering to learn the game).
And as for more modern games with "padding", going back to my Yakuza comment earlier - those games have become vastly easier to both 100% and especially platinum. Back in the day you'd be asked to pull off some pretty tough objectives to get either in Yakuza. Remember the Haruka baseball requirement in 1/1HD? for the PS2/PS3? Where you gotta hit 20 tough homeruns IN A ROW? In Kiwami its made drastically easier, with no requirement actually being difficult to accomplish.
How about those climax missions that are "tough" in Kiwami? Takes a couple of tries usually. In 1(HD), one mission asks you to clear a particularly tough long fight and boss fight taking 0 damage, no checkpoints at all. Good luck clearing that one.
Then you have 2/2HD on the PS2/PS3 which has a completion list that has you need to complete every baseball challenge perfectly. In later games its just get a decided point score, even 3's EX Hard baseball isn't really that hard, is actually quite forgivable really, since there's some margin for error.
And then you have Yakuza 6 and 7 which just outright removes the need to do the completion list. 6 lets you pick and choose while 7 does away with the need to do any of it at all. 6 especially has an insanely short platinum time (and actually, 100%ing the game is pretty quick as well, going by how it took much less time to finish compared to even 3 or 4).
And also for stuff like oldschool Mario? I'd say these days its way, waaaaaaaaaaaaaaay easier to 100% something like New Super Mario anything, or 3D Land/World. Those games are easy. Like, insultingly easy. In most of the newer Mario games you get maybe 1 stage that's hard, maybe a half dozen if your lucky. The rest I can clear first time with everything without dying.
100%ing Mario 2 (aka Lost Levels) asks you to clear the game 8! times to unlock worlds A-D (on the FDS version, they make it easier on SNES). And to unlock world 9 you gotta make sure you don't use any warps. Also, the FDS version doesnt let you continue from the last stage, but rather the beginning of that world, so worlds 7 and 8 will really test you since clearing some of those stages will probably take a life or three alone. Also, world 9 doesnt let you continue at all. Compared to that, clearing anything in 3D World or New Super Mario Bros U is a pushover.
Oh, and I'd argue Yoshi's Island on the SNES is way harder to 100% than any Yoshi that comes after it. Finding all the collectibles is tough, and making sure you finish with full health (30 stars) while finding everything, especially on a couple of those boss levels can be a challenge.
So no, I don't think newer games are no longer possible to 100%. Less fun? For sure, but not possible? Not a chance, you just need lots of time to waste.