Did they used to all be so volatile and have these massive max payout potentials? Is it more that the RTP is all going to a few players rather than being spread out?
The best phrasing I've heard over the years is
suitably rare - the profile has certainly changed in recent years.
If we look back at a decade ago - something like Immortal Romance was your high variance beast and Dead or Alive or Break Da Bank Again were your ultra-high variance beasts.
Immortal Romance has a 12150x pay for a five wild reel Wild Desire - but 4 wilds will be around the 1000x mark, and 3 wilds can potentially hit the 500x mark (or land 3-4-5 and pay zero ?). Beyond that, it's pretty rare to get above 500x - a substantial Wild Vine bonus perhaps.
Dead or Alive goes up to around 13000x for the dream bonus - contrast with 111000x of the modern successor.
Break Da Bank Again was all about the one big line hit - 5 gems, with wild, in the bonus was worth around 4000x. It was known as a spicy proposition and people would lower their bets when playing, fully aware a 15 spin bonus could pay literally zero.
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I would personally point to two events here - Bonanza with the 1 in 450 bonus frequency (which raised a lot of eyebrows at the time, when 1 in 150-200 was the norm) and then bonus buys as a whole. It's not uncommon now-a-days to hear of 1 in 500, 750 or 1000 frequency (every 40, 60 or 80 minutes at UKGC speed) - which makes for a miserably experience when playing slots designed for the bonus buy first rather than the whole experience.
When it comes to the streamers - not only do you have considerably more streamers, but they're spinning considerably faster - it wasn't unusual to see an old school streamer doing 500 spins an hour; when we're seeing 500x, 1000x, 2000x or more bonus buys... it's entirely plausible that an old school streamer would need
days or weeks to hit one - but the monopoly streamer can click a button.
Once the whole lure of "max win" (in my day, they didn't have max wins... ?) took hold, providers followed the trend and the incentive to push more and more RTP upwards for "streamer wins" - combined with the triple whammy of more progressive jackpot contributions and lower headline RTP (house edge increasing from 3-6% to 4-12%)... makes for a much more brutal experience.
Personally, I still play a lot of the older slots at their original setting (if I can find them) - although my experience with "streamer slots" has given me a renewed interest in watching paint dry... it's more interesting!