I'll say everything I have to in one post and disappear again. I have limited free time and energy, and I would rather use that towards doing things I enjoy and are more relaxing (like video games). So I apologize for that. This is a pretty nuanced and challenging topic, and the same pushback you see here happens with most of Epic's exclusivity deals.
Game development isn't cheap or easy, and these deals tend to offer baseline payment guarantees. That can help out with early funding and reduce the risk involved. No game is guaranteed to sell well, and nobody knows what Stardock's accounting looks like. Many developers I enjoyed have gotten absorbed or shut down over the years because they couldn't afford to keep making the games they wanted to. Gas Powered Games is the first that comes to mind.
Also, Stardock isn't a one-man-band or a massive gaming studio that's 100% immune to changing market conditions. There are a lot of people that rely on the pay they receive to support themselves and their families. Even if some people don't like the decision, this provides better financial stability for those creating the game. If these Epic deals existed earlier, other devs might still be around. Potentially great games never got the chance to exist because of a lack of funding.
Sins of a Solar Empire: Rebellion's peak players are pretty misleading. If you look at lifetime concurrent player stats on Steam across the past 10 years, it's floated closer to 1-1.5K players with some dips below that. It only had a few massive short-term spikes during two different free promotions and a humble bundle. I'd imagine a solid chunk of that activity isn't new players/buyers either. Stellaris sustains much larger ongoing player counts, and has frequent updates and DLC. They also have multiple large development teams, have a much bigger and equally loyal fanbase, and can fund everything internally. It's also a really good 4x game, which helps.
If people dislike Epic's launcher, wait for the full release elsewhere. It'll be a more complete game after early access is finished. It will likely end up on GOG and Steam eventually. If people are concerned about the game quality, check out gameplay videos and wait for reviews to decide. If people dislike the price, wait a few years and buy it at a discount. If people dislike Stardock for accepting a deal with Epic and think it's a deal-breaker, then don't buy the game.
Life is too short and complicated to get overly caught up or angry at a developer, person, game, or situation within this industry. At the end of the day, game developers are trying to bring something fun to others. Video games are entertainment, after all. They also have to make enough money to make that possible, and a deal with Epic is far less intrusive then some of the in-game microtransactions out there. I'm okay with the deal because it's the right thing for everyone who works for Stardock, not just Brad.
Unfortunately, none of this answers the question about how long the exclusivity lasts. Nobody knows, and the contract is likely too air-tight for anyone to even hint at it. There's a decent chance of it being one of a few things:
1. 1-year exclusivity, which is how Epic often structures these.
2. Full early access exclusivity, estimated at 1 1/2 years based on their FAQ.
3. 2-year exclusivity. So full early access + 6-month launch exclusive.
Longer than 2 years seems unlikely unless they offered a long-term deal that was too good to refuse. It's equally unlikely that it's shorter than a year.