My only guess is the remakes made Kanto easier to progress. My reasoning comes from the Elite Four having their pokemon levels lowered than in the originals.
I don't think Rhydon is that much powerful, is it? And I think the reason that the elite four and the champion have levels reduced is because rematch exists. Do you have some more reasons?
If I had to guess, it was to reduce the ammount of late-game grinding for the final fights of the game. I actually just finished a playthrough of Yellow, and when I got to the league I was anywhere from level 40-45, while the 8th gym already had levels in the mid 50s. Still odd they devolved it when lower levels would have done just fine.
There was something in the first three Gens called "Badge Boosts" which granted an invisible power-up to all your Pokémon stats by the time you had all gym badges.
Badge boost was 12.5% in Gen 1-2, nerfed to 10% in Gen 3, and completely removed from Gen 4 onwards. So even at two levels lower the FRLG Elite Four is probably still stronger than their RBY counterpart.
This is an aspect that many players (myself included) didn't know when playing Gen 1-2, leading to the wrong idea that grinding levels for the Elite Four was necessary to prevent being unfairly underpowered, when you can actually be overpowered in those games even when apparently underlevelled.
As for the Rhydon thing, I suppose it might have been just a mistake. Rhydon was never an issue to deal with as it got outspeed and destroyed by any Grass or Water move in Gen 1-3, or by any strong special attacker due to its garbage special Defense.
I know that you can become champion even if you're underleveled, because I watched some videos.
I also think it might be a mistake. Even if Rhydon can be defeated easily, it's presence feels strong. And just like Rhydon, any Pokemon who is rock and ground type can be defeated by any grass or water type attack.
In HG/SS, Giovanni only has four Pokemon, and doesn't have Rhydon and some other Pokemon. He could have fill the two slots, but just like many gym leaders in HG/SS, he just didn't. I think him not having Rhydon in HG/SS makes sense since he didn't had it either in FR/LG, right?
Where is Giovanni in HGSS? Isn't the gym leader Blue?
Giovanni is an event exclusive battle. Get a special celebi and you travel to the past to fight him
My only guess is the remakes made Kanto easier to progress. My reasoning comes from the Elite Four having their pokemon levels lowered than in the originals.