I think they were, at least in some respects. The way that the designers pulled out all the stops with the new legendary and mythical Pokemon (among them, the alleged creators of the universe), and the large selection of evolutions for older Pokémon, gave these games the feel of a "series finale". Had Game Freak decided not to continue the Pokémon series indefinitely, Sinnoh would have been the perfect region to end on. (It's interesting how Sinnoh was followed by a semi-reboot with Unova, much like how Johto - which was originally meant to be the end - was followed by a semi-reboot with Hoenn.)
There seemed to be a greater emphasis on completing the National Pokédex - catching them all - in Diamond and Pearl, compared to Gen 3. A lot of thought went into making all the Pokémon available in these games, with features like Pal Park, the Poké Radar and the dual slot method.
And last but not least, the introduction of the physical/special split, and online connectivity, broadened the scope of possibilites for competitive battling and multiplayer interactions.
Diamond and Pearl did have some issues that still vex fans to this day, such as the slow animations and the bad type balance in the Sinnoh Pokédex. The games were soon surpassed by Platinum, HeartGold and SoulSilver. But at the time, I think they were a monumental achievement. I can only hope that Masuda has remembered the passion and determination he felt when making them, and injected this into Brilliant Diamond and Shining Pearl as well.
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