Let's Encrypt currently plans to move away from cross-signed roots entirely (New Intermediate Certificates - Let's Encrypt), instead letting server operators choose (via alternate chains) their preferred intermediate to either ISRG Root X1 or ISRG Root X2. So yes, for the near term future you can always use trust paths to ISRG Root X1. For the long term future, we don't know if Let's Encrypt will still be providing this option in N years.