Higher quality lens, better sharpness, colors, edge sharpness, build,weather sealants and durability.
It all comes down to what you are shooting, the weather conditions, who you are shooting for, personal work or sales to clients. YOU ultimately decide what direction you want to go in.
Whether you want to be a hobbyist or a professional. When people are paying you to produce a certain body of work, and your gear can't produce that, then you'll know it's time to get better gear. IF your clients can't see the difference, then why change.
If you are merely doing photography as a hobby, your income will dictate your gear choices.