Reasons for modern era characters to be more powerful
I'm not referring to the actual evidence, .i.e. Lucas's comments about the Golden Age of the Jedi, the fact that Yoda and Palpatine are the most powerful to their times, yet others in their era are stronger relative to them than older eras were relative to weaker leaders, etc., but rather possible explanations for this. It does seem as though the presence of Force sensitive varies by some supernatural periods independent of demographic concerns, seeing as how it was said that the dark side was growing stronger over the course of the Banite sith, the Celestials were a thing, etc. But we can also look at more mundane reasons:
1. Training and techniques improve over time. The Jedi Order can obviously store information, so it's relatively unlikely that it would lose info faster than it would gain it. We can point to several direct examples of new inventions and innovations (.i.e. vaapad) over just the course of the movie era, so it's not difficult to imagine that those improvements compounded over 25,000 years. And it's unlikely that things hit a complete plateau, not only because of said direct counterexamples, but also looking at other things, such as technological feats, that clearly advance over the eras (you see no ships like imperial star destroyers in the time of kotor, for instance).
2. The population probably grows. Even with widespread birth control options, the richer parts of the western world still experience population growth IIRC, so I could imagine the same happening the SW galaxy, especially since many planets aren't super heavily populated. Moreover, in the thousand years since Ruusan, the Jedi probably have a much better infrastructure for finding Force sensitives, given the prosperity and lack of competition. So that means you have a higher chance of finding Force sensitives from all across the spectrum, and so you'll produce more outliers like Yoda.
3. Sidious and Plagueis in particular benefited uniquely from their midichlorian manipulation, which may have allowed them to boost their potentials past barriers earlier sith had naturally run into.
Now, there are two counterarguments commonly given; one, that lots of arcane techniques were lost, and two, that the ancient Jedi/Sith had to fight more wars. The former is partially true in the sense that sorcery and such were more common in the older days, although the newer times come with their own abilities, but I don't think that factors into overall Force mastery as much as you'd think. Take Palpatine, for instance; he was very knowledgeable on sorcery but just used more "conventional" techniques in combat anyway, as if by that level of raw power and mastery, such gimmicks were no longer as important.
The latter isn't as important as it appears either. What we're talking about - lightsaber combat and mastery of the Force - can easily be developed given the technological capabilities of the Star Wars universe without the need to actually go into combat. Likewise, modern UFC fighters are far superior in their particular area to any ancient army soldiers, and had we lived in constant warfare, we probably wouldn't have developed the resources and infrastructure to train them, ironically enough. You may say that there's less of a drive or impetus to train in the combat arts...except clearly the Order hasn't adopted that mentality, seeing as how an incredibly large portion of its training still involves just that. So excluding celestial like beings, I still see no reason to doubt the large body of evidence pointing towards the modern era being the most potent, on average, in galactic history .