If the other participant trusts X2, then X1 doesn't matter for these certificates. But, most people in this community provide services to either everybody or at least to general users who would just be trusting whatever roots their operating system, browser or other software trusts. This thread is about the (likely multi-year) process by which Let's Encrypt gets those operating systems, browsers and other software to trust X2. Until that happens to a degree that's satisfactory for you, the chain to X1 needs to be there or else these systems have no reason to trust your certificate.
From a totally selfish local point of view, having ECDSA end entity certificates already means your server doesn't need to do RSA, the chain is just a bunch of bytes which needn't be interpreted locally.
In web browsers and other modern sophisticated software, as trust in X2 becomes widespread (perhaps as soon as next year I think) these clients don't need to do RSA any more because they can conclude X2 is trustworthy despite being sent the longer chain. So for them the X2->X1 chain link is just a few hundred bytes on the wire extra wasted.
Eliminating that last waste is a compromise between the wasted bytes sent (a very tiny but real cost) and the inconvenience of older systems not trusting a system unless it chains back to X1. That cheap Android phone you bought in 2018 may never end trusting X2, so either your site just doesn't work with that phone or you'd need to keep delivering a longer chain.