I was with you until you said it made DP look good in terms of gameplay
Mmm, I know DP are extremely slow and had poor regional 'dex choices for the main campaign, and Platinum is significantly better in every respect, but there's a
lot going for them that Sword and Shield can't hope to match.
First of all, Sinnoh
really went all in on making absolutely every normal Pokémon available
except Legendary/Mythical/starter Pokémon, and also DP missed Tangela and Tropius but Platinum added them as well - and it wasn't even all through one singular, repetitive centralized feature, nor by cramming them all in with low encounter rates, but by adding a
ton of different ways to discover Pokémon in familiar places.
They had three different ways to change encounter methods (swarms, dual slot mode, the Poké Radar
which is still the most engaging Shiny hunting feature in the series don't @ me), two areas with their own unique sets of changing encounters (Trophy Garden, Safari Zone), a totally new mini game to get a thematically related set of Pokémon (the Underground... fun fact also-- this was the first time fossils weren't one-time key items)
and another seven routes and several more dungeons to explore with their own unique encounters.
On top of that, old Pokémon that had special gimmicks all to themselves - Unown, Feebas, Deoxys - were fully done justice, with Unown and Feebas both getting completely uniquely dedicated areas (Unown a sidequest on top of that) and Deoxys's awkwardly handled form mechanics being replaced with something new (how many people knew that it was Gen IV and not Gen III that set the precedent for being able to choose its form or having more than one form in a game?).
Obviously, the National Pokédex was much smaller at the time, and I can understand why something quite like this wasn't feasible again - in fact, I think the cut Pokédex is the
least of Sword and Shield's problems
don't @ me on this either and I don't really find transfers to be important to my own experience. But this isn't about
how many Pokémon were available - it's about
the lengths they went all so they could justify squeezing all of them in. As a kid, I literally filled my PC in this game
and didn't notice... I finally found that Skarmory I wanted to catch and then couldn't throw a Poké Ball ;-; because there were so many
things to do in the
process of catching every Pokémon - and I wasn't even trying to fill the Pokédex, just thoroughly
playing the postgame.
And then once you did get those old Pokémon, they also had new evolutions! Most of these weren't used for advertising (people were still discovering them after DP came out!). For that matter, they weren't even part of the campaign - which is obviously a bad thing in practice, and I'm glad Platinum fixed it, but it really goes to show that it wasn't by necessity or perceived obligation or to fill a quota or to suit the needs of the story...
they just wanted them to be there, so they made them.
I've already talked about why I miss crossgens and Mega Evolutions and why I really don't think Sword and Shield's approach is a valid alternative.
They also refined systems that needed polishing, like roaming Pokémon and the physical/special split; they completely overhauled Contests, Secret Bases and the Safari Zone to take full advantage of new technological advancements; they brought back day and night cycles even though those were
supposed to be a one-off gimmick for Gen II only; and they
introduced the Wi-Fi connectivity features that defined the franchise for the next four Generations
(may they rest in peace).
And that's not even to mention how utterly massive Sinnoh was as a region and how well most of its areas were mapped (compared to Galar...
whew).
Diamond and Pearl were extremely unpolished games held back by slow speed
to be honest I think the people who complain about this specifically are overblowing it a little, but I'm pretty patient in general, a poorly executed story
do I need to do my Sword and Shield story rant, too? because I will make that post, and, yes, a terribly selected regional Pokédex
yeah I'm not gonna argue this one... this is very correct and on a visual level, Sinnoh didn't look all that great before Platinum revamped it, either. But they had ambition in
spades - exactly the kind of ambition that's completely missing from Sword and Shield - and they
delivered on it.
It's true that Platinum was
very necessary to improve Sinnoh's
main campaign, and I can understand why Diamond and Pearl are so unpopular in that area without the improvements Platinum brought. But honestly, I think even Diamond and Pearl had one of the most impressive postgames in the series by a
great margin, and while they admittedly do deserve a good amount of the flak they get (and Platinum deserves all of the credit it gets for "saving" them), I think you're giving them too little credit - they give Sword and Shield much steeper competition than you think!