This may be the Gen 4/Sinnoh kid in me speaking but Diamond and Pearl are honestly not bad as a foundation. Yeah, compared to Platinum they are noticeably inferior, but they're still pretty fun to play through even if the things I've noted are worse, like most of all the expanded Dex is indeed quite big: getting to use all the Gen 4 evos in an in-game playthrough, and that also helped the enemy rosters for Trainer NPCs a lot, even route ones, since route Trainers in Diamond and Pearl are quite repetitive while Platinum's are a lot more varied. I also talked about it a while back but Platinum Sinnoh just plain looks better than Diamond and Pearl Sinnoh, much more vibrant and diverse and it looks more alive, while DP Sinnoh is pretty eh by comparison.
That said Diamond and Pearl aren't totally bad, the base Sinnoh roster has plenty of fun Pokemon like the well known Shinx and Starly, but also stuff like Mismagius and Honchkrow (who were totally absent in Platinum), Bronzong, Abomasnow, and whatnot. Not to mention the trade four are actually surprisingly fun thanks to new tools like Gengar actually having STAB off its best offensive stat, Rock Polish Golem, No Guard Machamp, and Alakazam with a more varied movepool. It's a pretty decent foundation, even if it was one Platinum needed to improve upon.
What I really find to be the downfall of BDSP though is that it can't even claim that it merely used DP as its foundation instead of Platinum. FRLG, HGSS, and ORAS used the base Red and Green, Gold and Silver, and Ruby and Sapphire specifically for their foundations, instead of Crystal and Emerald, but HGSS and ORAS integrated some elements from their third versions and most of all, at least tried to be more distinctive themselves. HGSS and ORAS are more modernized reimaginings of GS and RS, integrated with elements of DPP and XY, changing the experience to become proper Gen 4 and Gen 6 games respectively, like story and gameplay elements, and feel like very distinct and updated experiences. Like they're Johto and Hoenn in a new style, and I feel like I'm playing a different game from the original. They're a new spin on an old region, using a base game as a foundation but bringing their own unique changes that give them an identity of their own, separate from both Crystal/Emerald and the original base games of Gold and Silver or Ruby and Sapphire. FRLG has this too to an extent with a lot of new gameplay elements and a whole new area in Sevii.
No, what's truly the problem with BDSP is that it is literally a copy-paste of Diamond and Pearl in Unity with Sword and Shield mechanics sloppily implemented into it. It adds things like the Grand Underground, EXP All, and SwSh affection (without the friendship cap, might I add!), that completely destroy the balance of the original Diamond and Pearl experience while leaving the base experience completely and utterly unchanged. Diamond and Pearl, in its original incarnation, is not a perfect game, but if you play the original Diamond and Pearl you can at least claim it's going to work as intended, and you will play by its intended design, with the game design being coherent and one you can abide by. It's a relatively cleanly designed experience that can be experienced as the original designers intended for you to experience it. BDSP takes that design, copypastes it with no changes, and sloppily throws in certain modern mechanics that DP was not made with in mind without actually altering the experience to accommodate. ORAS had (optional) EXP All and had a different level curve to make it workable, same with many different and new Trainers being thrown in here and there with different rosters to boot.
That's not even to get into the Gym Puzzles in BDSP being identical, like the Snowpoint Gym puzzle, which was designed with a 4 way 2-D movement grid using a D-Pad in mind, but the Switch has diagonal movement thanks to its use of control sticks. Well if you try to use it here...good luck! You can be screwed! Potentially soft-locked if you keep autosave on! Meanwhile ORAS and HGSS both changed the Gym Puzzles to take advantage of the expanded capabilities of the new hardware, ORAS in particular was working with diagonal movement thanks to the 3DS Circle Pad and made new Gym Puzzles accordingly. HGSS was less substantial, but the DS was capable of more expression than a Game Boy at least, and you had a more expansive Kanto with better puzzles.
The fact that BDSP was an afterthought in lieu of Legends: Arceus shows a lot. Well, at least we can say Legends: Arceus is the proper Sinnoh "remake" in the vein of FRLG, HGSS, and ORAS, only that it's a new experience in ancient Sinnoh, but it integrates most of the Platinum roster including all of the Gen 4 evos, and has a post-game episode starring Giratina like ORAS did with Rayquaza, and we had two groups named the "Diamond Clan" and "Pearl Clan" in clear shout-out to the OG games being named Diamond and Pearl. That's what we can call the true Sinnoh "remake".