All other things being equal, Sonic would still be easier due to its rings health mechanic. If you were Sonic, you could just peel through world 8, rushing to grab at least one ring every time a bullet bill smacks your brazen face. As Mario, the injury lasts forever, in fact it fight cancel the power up you just consumed from the map inventory, punishing every small mistake in agonizing attrition; Sonic's survivability makes finding all those paths and enjoying the loopy level design easier without as much anxiety (so it's the right system for that game, but undoubtedly easier). The final boss to Sonic 3 just beat me senseless while I kept grabbing my last ring over again, that's how I was able to beat it in 1994.
Yeah the ring system does offer a cushion for Sonic that 2D Mario games did not provide. Although, there are times even in the 2D games (especially CD) where you can start a level with no rings and die before you come across one.
If you think about it, Sonic's design kind of seems influenced from the habit of quarter feeding people would do with arcade games, I wouldn't be surprised if Sonic Team and STI had that in mind when making the ring health system. So just like how you can be sloppy and quarter-feed your way to beat an arcade game (but spend way more money in the process), you can play sloppy and get by with just one ring in a classic Sonic game.
That's an option, but naturally most gamers want to get by with better skill so they'll replay to not blow through sections quarter-feeding, or get by a level in classic Sonic with just one ring. The classic Mario games have something of a callback to that type of arcade mentality too in the way power-ups would work to absorb a hit (at the cost of losing the powerup) like you just mentioned.
True. Sonic & Knuckles cutscenes were short and on point. They showed you exactly what you needed to be shown without wasting your time with extra fluff.
And this highlights a bigger issue with the game industry IMO. Because when storage changed to optical discs and developers had the freedom to "express themselves" and games became more "cinematic" (something people hyped to high heavens as if that's a good thing)... the storytelling got worse, not better (at least for any genre that's not a story heavy RPG). Because now you had these long cutscenes that waste your time, all the exposition and all the voice acting, which most of the times was bad and completely ruined the character's tone and personality (like it did for Sonic, IMO).
And games that were never supposed to be story heavy, now had to be, otherwise they were considered archaic design. I still remember reviews cutting points from games because they had a simpler way of telling the story.
The evolution of storytelling in videogames is the poster child of "less is more".
TBF in the best of games you can just skip the cutscenes if you wish, often even in a first playthrough. The cutscenes are there if you want them, but if you don't, just skip to the next gameplay segment. Of course, a great game would also still provide you ways to find out those parts of the story through exploring and playing the game anyway, even if you skipped the cutscene.
I'd say a lot of games did benefit from the expansion to CD-ROM that weren't just RPGs; you're probably more of a purely gameplay-driven person but for people who are about the experience of a game, those cutscenes can really help set the tone and atmosphere. Survival horror games wouldn't have the punch they got if it weren't from cutscenes gaining more a foothold in gaming, for example.
FWIW, that transition period had a lot of things we'd consider bad takes in retrospect, like Western media completely dismissing 2D games or games like System Shock being reviewed poorly because they weren't enough like DOOM (they were never meant to be like DOOM!). Some games did get the right lesson of using environmental storytelling rather than overdosing on cutscenes, though. Most of them were Japanese games though, and they were generally ahead of Western devs on that type of thing by at least a decade.