Many people buy games outside of Steam. Sure, relatively speaking it's a minority and if a game is available on Steam and elsewhere, most will pick it up on Steam.
But part of the reason why Steam is so good is because these other platforms exist and there's nothing actually stopping anyone from buying their games from other stores. Cloud saves, game streaming/remote play, online play, family sharing and many more features are all free/included with the game purchase on Steam and they also pioneered many of these features. Steam Workshop adds great value as well, there isn't anything remotely comparable on any proprietary console.
Steam is good because it has to be in order for people to choose to use it.
And "deep discounts" are the same as ever, I see some games 90% off on sales events. Sure, successful AAA titles usually don't get a big discount 2 weeks after release, but in the end the publisher sets the pricing anyway. Generally, even when comparing full price, games are just cheaper on Steam compared to PSN (10 to sometimes 20 $/€ for big titles).