This is one of the most common things I see when people say they like the Johto games. Johto fans often mention "Two regions!" as one of their main reasons for liking the Johto games. But I don't think I have ever seen anyone say "Two regions, and it's good." Which I personally don't think it is. It seems like some people prefer quantity over quality when it comes to regions in Pokémon. Personally, I think other games have better post-games in terms of new areas to explore, notably FR/LG, D/P/P, B/W and B2/W2. They might not have an entire new region to explore, but what they have is much better executed than Kanto in the Johto games. Apart from that, I agree with almost everything else you said, just wanted to comment on this.Being able to revisit Kanto - It's boring. Aside from battling the Gym Leaders and Red, there really isn't anything noteworthy to do. Kanto was already pretty dull in the original games, but in GSC, the region is about half its original size, the few locations that do remain are really just there for the sake of being there, the player still runs into Lv. 2 Rattatas on Route 1, and there's just nothing else new to look forward to once you get there. It seems to me like this iteration of Kanto was included the way it was just so players could reminisce about their trek through the region the previous generation - which may be a great idea for a museum exhibit, but not so great as a gameplay choice.
Regarding how G/S/C felt mysterious, that is something I actually agree with. My first Pokémon game was Blue, but my second one was Silver and my third was Crystal, so the original Johto games were a big part of my childhood. I definitely feel that they had an atmosphere of mystery which I don't think the remakes managed to recreate, though that is partly because of how you nowadays can find out everything you want on the internet, as green_typhlosion said. At least for us older players, I'm not sure if the children of today are able to do the same. If not, perhaps they can get the same feeling of mystery from the modern games that we older players got from the older games which we played when we were younger.
I also remember some specific things about the mysteries in G/S/C. When I was a kid, I managed so solve some of the mysteries in the games on my own, though I had two guidebooks for the Johto games (one for G/S, one for Crystal). They taught me some things I would probably not have figured out on my own, such as where and when you can find Lapras. Though I'm not sure if they featured everything in the games, I don't remember if they even featured all Pokémon or if they showed where to find Lugia/Ho-Oh. Sadly, I do no longer have either of these guidebooks in my possession, so I can't go back and check. That said, I remember that I was able to figure out how to get to the cover legends on my own (partly, at least), which is one of my best memories from G/S/C.
I also disagree with the statement that HG/SS fixed the issues with G/S/C. If anything, I feel that they made minor improvements at best, while keeping most of the flaws from the originals. Notably the level curve, the Pokémon distribution, the terrible story and the lackluster post-game. I played G/S/C before HG/SS, and I remember that when HG/SS were released, I was heavily nostalgia-blinded, thinking they would be the best games ever. But after getting them, I mostly ended up disappointed. The more I played them, the less I liked them, and the blindness caused by the nostalgia eventually wore off. In the end, I did not like HG/SS all that much. They are my least favorite Pokémon games for a good reason. In comparison, I actually like G/S/C better. They are far from perfect, but they were at least fun for their time, which I can't say about HG/SS.
I think I have stated my opinion on everything else regarding HG/SS in earlier posts in this thread, so I won't do it again.
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This would have been a perfect opportunity to include my upcoming unpopular opinion about the Johto games, but sadly, it is still not quite ready. I have been working on it a fair bit lately though, so I should hopefully be able to post it soon.