Outsourcing endianness pain to your customers is an easy way to teach them about segfaults and silent data corruption. s390x is niche, endian bugs are not.

Network protocols and file formats still need a defined byte order, and the first time your code talks to hardware or reads old data, little-endian assumptions leak all over the place. Ignoring portability buys you a pile of vendor-specific hacks later, because your team will meet those 'irrelevant' platforms in appliances, embedded boxes, or somebody else's DB import path long before a sales rep waves a support contract at you.