Why mention Platinum as a game with tedious grinding. If anything, Diamond and Pearl are much worse, especially for the Pokémon League.
Why mention D/P as games with tedious grinding when you have HG/SS which feature extremely tedious grinding all the way from the beginning to the end, during both the main game and the post-game? Say what you want about D/P, they did at least have a functional level curve and a way to rebattle trainers efficiently, two things HG/SS desperately wish they had.
On the current topic of the new Exp. Share and difficulty in Pokémon games, I have some things to say. I did not read through every previous post on the subject, so apologies if I'm repeating things others have already said.
First of all, I dislike how the entire party is forced to get Exp. in Gen 8. I generally try to keep my team members at an even level when playing through the games and this made it very difficult since all of them always got Exp. after every battle or capture. This was especially notable when I added a new member to the team. And due to the way I play, I started getting overleveled after the third gym, with no way to turn back. The only solution I saw to this was to train more than six Pokémon. My team in Sword ended up consisting of 15 Pokémon in the end, which made my playthrough more balanced. I only got underleveled in the end of the game, against Eternatus and Leon. During the rest of the game, my team members were at the same level as the opponents' Pokémon or slightly above. While this was admittedly fun and functional, I still think that Exp. for the entire party should have stayed optional like it was in Gen 6/7.
Which brings me to my next point. I like the way the Exp. Share works in Gen 6 and 7. I thought it was well done, and best of all: it was optional, you didn't have to use it if you didn't want to. I had it on when I played through X since I wanted to give it a try, but this led to my team getting ridiculously overleveled. It was at above level 80 when I beat the game. In comparison, Diantha's Gardevoir is at level 68. I turned the Exp. Share off during my playthroughs of Y and OR/AS which led to much more balanced experiences, I only turned it on to make grinding easier at some points during the games, most notably before the E4. I kept it off for S/M and US/UM too but had a vastly different experience with them, more about that further below. Also, I disagree with those who think that the Gen 6/7 Exp. Share should only have been available after beating the E4. If you don't want to use it before the E4, just turn it off. It isn't that hard. In fact, it is very easy and only takes a few seconds to do. On the whole, I'd say the Exp. Share in Gen 6/7 is an example of good game design because it is optional, the same can't be said for Gen 8 though since it is not optional there.
As for the difficulty in Pokémon games on the whole, I don't think the series has ever been that difficult. If you are looking for a difficult RPG, you should play something else than Pokémon. Or play Pokémon, but with a self-imposed challenge. While one might argue that the games in the newer generations have gotten easier, they were never really that hard to start with. If someone finds any older Generation to be harder than the current one, I don't think it is because the games have gotten easier (at least it is not the only reason), but because we were younger and less experienced when we started compared to now when we have a lot more experience with the games and have played several games/generations. But there is one exception.
I found Gen 7 to be surprisingly hard (at least with Exp. Share turned off). Both S/M and US/UM, they had an unbalanced level/difficulty curve as they featured several sudden level "jumps" during the main game, with pretty bad grinding spots right before these jumps. And my unpopular opinion is that I did not like this. I never asked for a harder Pokémon game and I wasn't happy about it. The Gen 7 games also had another thing I disliked in terms of difficulty: they introduced several "unfair" battles where the opponents have advantages over you. The most notable are the Totems, the SOS mechanics and Ultra Necrozma. Those kinds of battles are common in many other RPGs but Pokémon had done just fine without them IMO, there was no need to introduce them. This continued in Gen 8 with the Max Raid battles and Gigantamax to an extent. And I don't like that. I remember that when I got to Ultra Necrozma in Ultra Moon, I got angry because this was not what I wanted to see. As said in a previous post, I partly struggled to get through US/UM and this did not make things easier. Fortunately, I was able to beat it even if it came at the cost of a Focus Sash and required a bit of luck. I also disagree with anyone who claims that Gen 7 is easy even if you play with Exp. Share off, that is very different from my own experiences and I am very curious about the teams and playstyles of those who make such claims.
Lastly, since Challenge Mode in B2/W2 was mentioned earlier in this discussion, I might as well share my own experiences with it. I played through White 2 on Challenge Mode and I did not really find it more difficult than usual (at least not much). However, I did not find that it required more grinding either, I was on the same level as the opponents during the majority of the game. Due to B2/W2 having a great level curve, the new Exp. system and a free Lucky Egg, my team members always got a lot of Exp which made them grow quicker, requiring less grinding. I think White 2 on Challenge Mode was one of the most balanced playthroughs I have ever had of a Pokémon game, it is a prime example of good game design if you ask me.