so, which Big Tech company do you think is going to shit the bed next and popularize its Fediverse/FOSS equivalent in the process?

I still like IRC and I’m surprised that it got almost completely murdered by Discord.

I just hope this is the start of an internet renaissance with less corporate control.

It'll be hard to get people to not only detach from something they're accustomed to, but also then attach to something unfamiliar.
I tried and am trying again with Mastodon, but a lack of users I wish to follow, a more confusing premise at times, and just overall more enjoyment overall (if that) with twitter as a platform makes it a challenge.
Lemmy however has checked all the boxes. It literally feels exactly like Reddit, and honestly like a fresh start to avoid the various decisions both Reddit admins and the community itself made along the way. I'm hoping more for the latter experience than forming when diving into the fediverse, but my above statement is most likely applicable for a wide sample of people out there.

Discord is doing a lot of stupid things lately i must say

God I hope so. Discord works fine as a voice chat and groupchat for games. But it's insane to me that people use it as a replacement for message boards or websites and hosting files. It isnt indexed so you cant google it and a groupchat is a terrible format for this. Even as reddit dies you have some people acting like a glorified group chat is a good alternative. As an addition and supplement to a message board or website community sure this is how it's always been even in the old days there were boards with an active IRC chat. As the replacement? Awful.
Teamspeak has a pretty slick new version that looks very much like discord. Not fediverse but pretty easy to self-host.

What about YouTube?

I looked online and there seems to be PeerTube at least.

I would love for something to replace YouTube, especially something in the Fediverse, but video unfortunately has much higher storage and bandwidth requirements, so it's hard to imagine that not being totally cost prohibitive at high levels of traffic, even if it's split across so many different servers. I'd love to be wrong on that, though.

There isn't much left.

First Facebook with their whole meta thing, then Imgur deleting all NSFW content and images uploaded by non-registered members, afterwards Twitter and now Reddit.

Twitch made a big mistake with their new sponsoring rules, but seems like they are reverting / changing it again due to bad community feedback.

Discord had a few changes the community didn't like, but nothing ground breaking yet. But they get more and more greedy and their platform is filled with scams, hackers, bots and sadly many bad people like child predators and content which Discord support does nothing against. They seem not to care.

YouTube, well, I think they might be next actually. More and longer unskipable ads, restricting or demonetizing many videos, bad communication with their creators and less rewards for smaller creators. In addition, they might put high quality resolutions behind their already existing expensive subscription paywall. There isn't any competition which is urgently needed.

UPDATE: Bad news about YouTube continues. Just now, YouTube Ordered ‘Invidious’ Privacy Software to Shut Down in 7 Days.

Which other big social media platforms are left?

The problem with anything video is still that it costs way too much to host, unless you're a giant who already has their own data centers and massive data pipes. You can't just throw it on a cheap VPS like text-based services
- Are you thinking of it as a centralized replacement to YouTube? If you're centralized, yeah, you probably need a data centre the size of Malta. There are decentralized alternatives (like PeerTube) where the cost is also distributed. If you're using PeerTube, you literally can "just throw it on a cheap VPS", and lots of people do, with no problems.
  I think the real reason decentralized video isn't going to catch on is because video (and YouTube in particular) has not been a community thing for many years now. There are very few YouTubers who make videos to build a community or connect to a community. YouTubers are on there for money, and there's really no alternative that can both host the videos and pay out big cheques to content creators.

I honestly don't think the fediverse will become nearly as popular as many seem to.think. It's still complicated to use/understand for many non-tech enthusiasts, and in the case of Reddit, while people are angry, I doubt most of their users are going anywhere any time soon. Some will leave, but it's not going to be a small number.

We keep going on about how Reddit relies on it's "creators", without whom they'll die. Frankly, a lot of the highest rated content is just repost of old videos or tiktok videos. A lot of that stuff isn't original, and the deep conversations are, in my opinion, few and far between. Sure there are some communities whi h have this, but they're not exactly over represented.

I don't have statistics to back this up, but I'd be willing to bet an entire doughnut that most reddit users have never posted even a single comment. People with that level (dis)engagement aren't the type to seek out alternatives. They just kind of drift away.

The day I don't see "join our Discord" where I would earlier expect to find "visit our forums" will be a good day.

A bloated live chat monolith is not what I want to use to discuss game bugs or podcast episodes.

Agreed. Live chat has its place for certain things, but for other things a forum type interface is better suited.
Revolt seems to be to Discord what Lemmy/Kbin are to reddit, but I dont see most people bothering with it unless discord makes some reeaaallly huge mistakes to piss the community off.