Why is "modern design" automatically assumed to be a good or preferable thing?
Older design was often or even typically not well-informed by actual understandings of how to design things to meet specific goals. It often didn't think about what actually worked or how it worked and frequently just indulged a creator's whims mechanically, or arbitrarily did things a certain way because the creator just made assumptions about how it could be done. Often designers didn't understand the math they were working with very well too (I think this is part of why d20 and d100-based systems have typically survived better, because the math is more straightforward).
You can like old design, right - a lot of older RPGs still work pretty well (or have been updated so they do) - but it's different approach, it's one that more driven by instinct and guesswork and less by careful thought and understanding of all the possible different ways you could go. We have so many more mechanical approaches to draw from in 2025 than we did in say, 1985, and since about 2010, designers have been more and more conscious in how they design systems to meet their goals (really it starts earlier, but it's by 2010 that it's become a big part of TTRPG design).
Also, it doesn't have to be preferable for
you, frankly.
Nothing in my post says "U MUST AGREE NEWER IS BETTER!!!!" does it? So if you want to argue with that sentiment, go find to someone else to argue with! Sorry! But I'm expressing a desire to see someone at least
attempt to do that, which is, AFAIK, something we've not even seen attempted (correct me if I'm wrong - there may well be RPGs out there who attempt this but that I'm not aware of - but certainly most modern RPGs using simulationist principles seem to be using certain approaches "because that's how it's been done" rather than stopping to say "but why?"). Nor I am making an "automatic assumption", btw - quite the opposite - that's part of why I want to see an attempt at this! Will it be better? Can simulationist approaches benefit from modern design approaches and the much larger and better-understood toolbox we have now? Maybe they can't! But it is, AFAIK, an untested hypothesis.
EDIT - Also, let's be real - Daggerheart, Shadowdark, Draw Steel, and a lot of D&D variants are clearly pretty damn good at achieving their goals, design-wise and creating fairly compelling packages. I want to see someone do the same for a simulationist RPG,
if it's possible.