I thought I was pretty clear. It would be very hard for a team to eat $4m cap, and get nothing for it. Buyouts are already hurting teams. It was kinda a crappy idea, and they will possibly double down on the stupidity next CBA.
You've moved the goalposts again (from SJ wouldn't retain on Burns, which you've never offered any evidence for). I disagree with "it would be very hard for a team to eat $4M cap" as well. No one wants to eat cap but it has different implications for different teams. If you're not near the cap, it doesn't matter much. If you are, it can be a killer.
You didn't even know that the most a team can eat is 50% so I don't think you've thought about this issue much. (That's a very basic part of the issue.)
If the SJ Sharks are no longer contenders and if they want to trade Burns and if they can get Burns to agree to the trade or find a worthwhile trade among the three teams Burns listed (which, IMO, is going to be the tough part if you assume they want to make a good trade, not the cap/salary retention part), they will probably not be that affected by retaining. It will hurt but $4M is typically not a big deal for teams who aren't contending unless they've managed their cap poorly or unless they've loaded up on bad contracts in exchange for picks/prospects (but then they aren't likely to be the team looking to eat $4M in cap).
Drafting/development/trades are going to be a much bigger issue for the Sharks than any retention on Burns' contract IMO. (Tough to draft well outside the top 10, esp if you're as risk averse as DW, and the Sharks can't afford to be inside the top 10 - esp the top 5 where you really win - on a regular basis.) If they are trying to rebuild, they'll need younger players. Those players have lower cap hits bc you don't get paid in the NHL for your RFA years. If they have more young players than they do now, cap is not likely to be a huge issue. If they are trying to contend at that point, retaining on Burns could be a big issue for them. That seems unlikely to me (going by our current roster/prospects) but it's possible.
I'm not saying cap isn't important. I've said it's always important ITT. I'm saying that cap or cap retention is a much bigger deal for some teams than for others and that what stage of growth (full on tank, partial rebuild/retool, working on rebuild that's mostly set, competing for Cup, trying to stay relevant) your team is at has a lot to do with how important cap and cap retention is.
Edit to add: I agree with PF that the Sharks probably would not need to retain 50%. It's possible but I agree that 25% is more likely, if they decide to trade him and need to retain.