What "business" do you think Let's Encrypt would lose? They're a non-profit offering certificates for free. While I'm sure they'll be happy to help if they can, I don't think that there's much they'll be able to do. Since the X3 intermediate (cross-signed with DST Root X3 version) expires in March it'd be tricky at best to issue a 90-day-certificate from it at all a this point, and that's assuming that the key is still available somewhere to do so.
There's some advice in this thread about what certificates to put in an embedded device trust store, including that you definitely want more than one CA in there (possibly one you create yourself) just in case one CA isn't available or has a problem.
But if you only put Let's Encrypt Authority X3 in your trust store, with no plan for when it expired, I'm really not sure what else you were expecting? The suggestion to get one from another CA would be if you also had a root certificate from some other CA in there, you could use that.