Also, having played the two games right before and right after DP but not DP themselves, I think this answer might be a bit exaggerated. First and foremost, the experience gaining and level gaining mechanics absolutely did not change at all between Generations 3 and 4. I don't know who said it did.
If Volcarona can easily sweep every Elite Four member in BW2 by itself with an 85% accurate fire blast, then Infernape and Staraptor can do it even more easily in DP. I know Cynthia is stronger than she is in Pt, but if you call dragon rush and giga impact "competitively viable" then you aren't playing enough competitive. If your team was around level 50 when you battled her, then that means you either used too many Pokemon or didn't battle enough of the trainers in the optional areas. So you can only blame yourself for that. And just because ice type moves are good against Garchomp doesn't mean you need an ice type Pokemon to use them. An ice beam Floatzel works great. It even outspeeds her Garchomp. Also, I don't know why you think there are no powerful Pokemon available before challenging the Elite Four. If Infernape and Staraptor aren't good enough for you, then I don't know what is. I'm pretty sure DP had better Pokemon than all the games before it, mainly because of damage categories. Back in GSC, the strongest Pokemon was Feraligatr, a physical attacker with no physical STAB. Generation 3 was a little better, with Swampert being ground type and Blastoise having good special attack, but they were both a little slow. All the fast starters, the fire types, had to wait until their level 30's or 40's to learn fire attacks that weren't ember. And in DP you have Chimchar, with amazing stats in speed and both attacks and learning both fire blast and focus blast before the 3rd gym. DP also introduced Starly, probably the most broken non-starter at the time. Sure, Spearow was great in GSC, but its best flying attack was peck until you get the HM02 fly. Starly learns wing attack almost right after it's caught. Then it goes on to learn both close combat and brave bird by leveling up. The addition of damage categories also helped a bunch of old Pokemon, like Gyarados. So maybe DP was harder than the other games, but I don't think the difference is that much.