EU3 went a major step into "sandbox" territory as opposed to "historical grand strategy game". It was perceived, at the time, as being as big a departure from EU2 as EU4 from EU3; just as you now find the same flaws with everything from CK2 onward (as I do). Here is a post by ubik (!) that gives you a glimpse of the argument:
https://forum.paradoxplaza.com/forum/threads/is-eu3-better-than-eu2.348269/page-2 Also featured: players complaining about PDX selling patches as "expansions" in 2008. Stat crux dum volvitur orbis...
But I admit that with said "expansions", Gen 3 PDX is only "decline" in the sense of "ruining my purestrain genre". Final-form EU3/HOI3/Vic2 are all fine games I still play. And privately mod, to align more with historical plausibility.
FWIW, I do not want a "remastered Civ 4" from Civ 7; as mentioned I think "Civ 6 but the AI is good" would actually make for a fantastic Civ 7.
Prejudice against mods comes from my self-loathing "libertarian" tendencies (counterbalanced by a strong aversion to "historical materialism"). As in: I love anyone for rejecting corporatist "wealth" structures and attention to "economy", and yet I immediately also distrust them for it (because where is the "existential" pressure? Sometimes it's still there, but it requires extraordinary, religious inward "motivation"/guidance -- so as not to give in to sin, excess, leisure -- and build only what is truly necessary. I realize this is an argument of faith much more than reason I require here. Essentially my exegesis of what it means to "make friends with the unfair Mammon".)
Certain mods seem to actively violate my (or Sid's, or Soren Johnson's) vision of strategy game design. (To illustrate, even better than Civ 4 design-wise to me is Go.) I had a specific one in mind as egregiously exemplary when railing against modding up there. You can probably imagine which one. But of course, cultures and creeds will constantly clash. I have no more authority in your eyes to declare something good or bad than what you award me for your own reasons. I do not believe there is a "dialectical" process in which my opinion must participate -- more in an endlessly-recurring storm. But enough of that.
It could certainly be that Realism Invictus is a far superior game to Civ 4, but these days, I would rather become good at a game I'm already familiar with
and which has fantastic communities of scholars attached to it (e.g. Strategy & Tips). Of course RI might have its gurus just as well, but given that Civ 4 has stayed the same for over 15 years while RI is still in development, it is almost by definition impossible for the knowledge bases to be on the same level. And I do not trust myself to have the intelligence to figure out groundbreaking RI stuff myself any time soon. To wit, in another turn-based strategy game I've played quite to my exhaustion, it took me about 5 years to start being able to innovate and "win".
PS. Even though I've also been playing since 2005 (as a kid on Settler having even less of an idea of strategy than I do now), I have not won, or even played, Civ 4 Deity yet. Recent efforts in Nobles' Club are meant to get me there one day.
Do these answers satisfy your interest?