I'm not--at all--saying that 4e's power list wasn't bloated (though there are reasons why your numbers are suspicious to me, that I'll get to in a moment). I am, however, saying that the strident insistence that "all Martial characters should draw their powers from the same Martial-only list" reflects an excessive belief that a certain meta-aesthetic ("one-stop-shopping," centralization, whatever you want to call it) is inherently and axiomatically more important than any game design considerations that might apply. That is, these assertions are made without context, plan, or anything like caution; they are instead asserted immediately, as obvious and unalloyed goods, without any (stated) thought to the potential negative consequences.
Yeah, as I said, there's actually no specific fundamental underlying principle here. I just remember the day I cracked open a PHB1 in 2008, and as soon as I grokked what they had done, THE very first thought in my head was "this isn't scalable." I mean, I honestly don't even comprehend how anyone in WotC thought it WAS and why they made such a blazingly obviously bad strategic decision. I mean, it MIGHT almost work with 5e spell lists (as it did with AD&D ones) simply due to the fact that so very little additional material is planned, the lists were intended to BE complete as written. 4e was never intended to be so, it was designed, clearly and undoubtedly with near-infinite expansion as a central tenet.
Ironically, I might be able to get away with class power lists in my game, simply because I would be unlikely to create lots of expansions, but I don't even have the means to make 500 powers for my own use, so its moot. I contend that not categorizing powers is a superior design choice, but merely because it works, and because it probably would be more robust overall.
But, again, beyond that, there are other considerations in my game. You can only ever acquire any game element by means of narrative logic in play in my game. There is no process akin to 'XP' and 'leveling up' in which you go to some list and attach new stuff out of the blue onto your sheet. So, any of the issues one might bring up in terms of a single list that are relevant to that sort of process don't apply. Knights can get fireballs (well, maybe, play to find out) but they won't get them because a player picked them off a list and thought they were the mechanically best optimized option. It will happen because he learned the secret of fire while captured by the Azers, or something. Sure, you could go and deliberately get captured by Azers to get fireballs, go for it!
There are, in fact, only a very few prereqs for anything in my game, and those are simply based on mechanics, you can't utilize "bond with companion" if you don't HAVE a companion, which is a feature that only normally appears on certain classes. Maybe you can "multi-class", it would rely on the same sort of narrative logic process (nobody has ever tried, so far, so I don't have a rule for that).
Now, as for those numbers themselves: how on earth did you achieve that? Even if you restricted character options down to, say, 12 classes (about half of 4e's actual set) and made only 3 builds apiece on average (about 1 less than my rough-and-ready average of 4e's actual options) and eliminated all themes and made it so there were only an average of 2 paragon paths per class with zero PPs for anything else (such as race), and only offered one ED per power source, and only offered one specific power per build (including Utilities) at any given level where powers are offered, and only offered say 20 races (far lower than actual 4e)...well, let's see. All of these numbers are minimums, by the way--some PPs and EDs get more powers through their features.
Per class: 60 powers
Per race: 1
Per PP: 3
Per ED: 1
12*60+20*1+2*12*3+1*3 = 720+20+72+3 = 815
So, yeah, I'd be real curious as to how you managed to cram anything even remotely like 4e's diversity of builds and options into only 500 powers, given how many things "powers" were for. Because I didn't even touch on anything like items that do something (each of which will have its own power), basic/universal powers, themes/backgrounds that might offer powers, etc.
(Also, I'd like your cite on the 50k number. All evidence I can find--including stuff I myself have said about this topic in the past, when the digital tools still existed--puts it closer to a fifth or sixth of that amount, between 8k and 10k.)
OK, so HoML has around a dozen classes. There are only 'builds' to the extent that you could pick from a couple class features. So, lets say that totals out to 30 builds maybe? I don't technically have PPs and EDs as such, there are boons which can effectively emulate those, but EVERYTHING is boons, so its hard to say in detail what is analogous to a 4e ED, PP, ritual, feat, or item, exactly. Characters advance 20 times before they reach the capstone of being epic (20th level). That alone cuts the power lists by 1/3rd. I think we've pretty much reached the roughly 500 level already, by your numbers.
Honestly, powers TEND to be associated with power sources. That isn't COMPLETELY true, but it accounts for a lot of them. There are a small number that are effectively class-specific, as they relate to features of a class (there are for instance a series of powers that relate to the beserk state, which only berserks have, so nobody else will use them) A lot of 'core' powers however are pretty widely used. That includes some basic martial stuff, some basic elemental powers (which anyone who wants to blast things probably wants), etc. Priests and shamans and whatnot ALL cluster in the 'spirit' list, along with witches/warlocks.
If I really fleshed things out to the hilt, could I have 800 powers? I guess, maybe. I can't see the game growing beyond that sort of level though. I really stay away from the 4e 'sin' of having 18 of basically the same thing but with some really minor variation. Clearly there ARE some pretty similar powers, but they at least have considerably different thematics, or different risk/benefit (yes powers can be risky) profiles, etc.
As I said when talking about bonuses up thread, I don't believe in hair-splitting trivial differences. I want things to be distinct. If something occupies a specific niche, then I am not that interested in filling the same niche three more times. When you play and you do something, people should be able to say "Oh, look X happened!" It is just part of the larger-than-life kind of 'epic action' that is being aimed at.