What real, non-theoretical, "would stand up in a courtroom" risk is there if the SSPL extends to cover the Linux kernel?

SSPL: you have to share your source code to the Linux kernel if you sell Elasticsearch-as-a-service"

GPL: you have to share your source code to the Linux kernel if you release binaries

SSPL+GPL: you have to share your source code to the Linux kernel if you sell Elasticsearch-as-a-service or if you release binaries

As long as the source is duly published under the merged SSPL+GPL conditions above, anyone trying to bring a GPL case against Elasticsearch-SSPL-on-Linux-GPL may discover that their judge refuses to enforce the GPL's hostility clause, because the SSPL strengthens the intent of the GPL without weakening it in any respect. We'll never know until someone gets to a judge, though, but my armchair speculation bet is that the "can't combine this license" clause won't hold up when the combination increases the total amount of Copyleft in play without decreasing it in any regard. I hope someone takes this to court and doesn't settle, or else we'll never find out :)

(I'm not your lawyer, this isn't legal advice, etc.)

No offense but this advice is rather risky and legally dangerous, please don't tell people to potentially put their products in jeopardy by doing this. The correct thing to do is for MongoDB/Elastic to just clarify the wording in the license.

No. The correct thing is to adopt a license I understand.

AGPL is okay.

I'm not hiring a lawyer when I'm selecting a few tools to benchmark. I'm not using EC until I've had the license reviewed by a lawyer. Ergo, redis or similar.

If the license is so badly worded that nobody can understand it, it needs to be corrected. I agree you should not hire a lawyer for that, it's not your lawyer's job to fix some other company's license for them. It's a shame that MongoDB appears to have completely lost interest in drafting SSPLv2.

Redis is still FOSS under the BSD license. If Redis Labs were to change the Redis license, AWS would fork it in a heartbeat. To help clear up confusion you can read about the Redis, Redis Modules and Redis Enterprise licenses here: https://redislabs.com/legal/licenses/

ElasticSearch know exactly what they are doing here.

By leaving it grey and a little open to intrepretation they know that AWS lawyers will have to advice AWS to avoid it because it might open them up to legal attack.

You write as though the AWS lawyers' advice is binding. However the article starts with a trademark violation and apparently in that case, the AWS lawyers didn't have the upper hand internally.