So... don't use it? I probably don't use 99% of the functionality in the software installed on my computer.
It sounds like you just have an axe to grind with anything relating to cryptocurrency.
AFAIK Keybase's clients are all open source. If this is a real problem someone (maybe you?) will fork them and remove the "cryptocoin crap".
What is it about this change that makes you assume you're going to have to deal with cryptomining et al?
I know a bit about how Stellar works, and I can't imagine any kind of software they could "inject" in their apps besides perhaps a way to post a Stellar address into the current merkle-tree they host and a way to fetch that through an API. How is that worse than having a Bitcoin address optionally includable in the same way?
The rest of their announcement, I legit don't care about. This is the deal-breaker here. You don't need a Big Announcement or to hire more people to add a Stellar address for verification. I know a bit about Stellar as well (granted, you probably know more) but this reads to me as a let's-be-a-wallet game.
And this is worse than having Bitcoin addresses: active functionality versus passive functionality. Difference of kind, yeah?
If they wanted to build some whateverthing that leverages Keybase for identity? Sure, go nuts, I think it's silly but it's not my time. But injecting it "into their apps"? No. These are tools for production, for me, and were developed with certain explicit and implicit promises. I see cryptocoin junk being shoved into production tools as a break of those promises.