Part of the problem is what is acceptable has shifted. Game sizes have gone up. And the tournament focused mindset has damaged any kind of immersion.
I don't know if it's covered in the video, though a top comment mentioned it, but it bares repeating.
1. The standards have gone up. There's a video somewhere where someone paints a primaris marine using retro paints and techniques, and the results are not pretty. Cool, but surprising ugly for someone that does mini painting as a job. Yes, it's easier to get those results than in the 90s, but it's still a higher standard than some are capable of.
2. Model count and game size has increased dramatically. The whole 1000pt army could be as small as 20 models. (Space marines could take combat squads, which were 5 man tactical squads) compared to the 2000pt standard today. Fuck, I remember Apocalypse rules which were designed for obscene game sizes and allowed you take titans. Now titans (knights) are basically a stock 40k faction.
3. People don't care about a "story" or "immersion". We never used those terms back then as it was a given. Supposedly Horus Herasy still maintains that lore-first mindset. In modern 40k, you're playing to win. Why does it matter if your army is coloured in right. There was a lot of unpainted and proxy going on back then as well, so I don't know how bad this problem is, if it's a problem at all.
1. Standards have gone up, but the number of people with 20+ years of experience painting has gone up. That said, I see better shit coming from people who have never painted a mini before given some proper instruction, compared to what people tried to decipher from shitty photos in magazines and books 20-30 years ago. Unless you're actually physically handicapped, you can easily get a better result than most did in the 90s(with far better readily available instruction), and the video covers this.
2. Who was only owning exactly 1000 or 1500 points and didn't have more minis than that years ago? Sure, there's definitely more minis on the table in a 2000 point game now than there was even in a 2000 point game 20 years ago. But you still owned more minis than what you could fit into a list at any given time if you weren't just starting out. Yeah, if you work your way up to 2000 points over a year(or however long it takes), not only will you get better at painting(point 1 again), you'll probably own more than that after another few years unless you're like that tranny who paints random shit and can't put an army together. No, knights are not the equivalent of titans. If anything they're closer to just being the equivalent of an army of guard vehicles.
Leman russ battle tank. T 11, 2+ save, 13 wounds, 185 points.
Knight preceptor(the mid size chassis), T 11, 3+ 5++(vs ranged only). 365 points.
So the knight costs just about what 2 leman's do, has the same toughness and wounds as 2 lemans, and actually has less firepower than 2 lemans, has a 5++ ranged only but the leman russ' get a better base save, and the 2 tanks would require target selection to kill vs shooting at single target. The larger T 12 knight chassis would be more point efficient except both of them are kinda shit and still have fewer wounds than a pair of T 12 rogal dorn tanks.
Meanwhile an 1100 point warhound will likely eliminate the 2 targets its shooting at once getting into LOS, but then being t13 and 40 wounds at 2+ 5++(again ranged only) it's now out in the open and can easily be destroyed in a single round(definitely within 2) so there goes more than half your army all while having less firepower than 3 medium knights or 6 guard tanks. Nevermind how much of a waste it would be to have a titan doing an action on a turn. The other 3 titans aren't even playable in 2000 point games(plus would end up occupying an even larger percentage of your army to just get blasted off the table in 1-3 rounds).
3. This is a shitty argument. "In modern 40k you're playing to win" who was ever playing to lose? Who plays any game to lose unless you're intentionally throwing the game because it's some little kid or you're more focused on teaching someone how to play. Do 30k players play to lose? Story and immersion certainly don't have anything to do with playing to lose. If I go to a narrative event that doesn't use competitive terrain layouts and scoring, I'm not showing up just to lose. And yeah, unpainted armies looked like shit ages ago, just as they look like shit now. Proxying things doesn't have anything to do with playing to win or lose or models being unpainted. It's not an issue at tournaments(and most narrative events) due to having painting requirements. Hell even taking a warhound to a 2000 point game, I don't expect to table my opponent and winning is likely going to be more difficult than if I didn't take it. But of course I'm still playing to win.