What's actually broken about it?
If it's mixing bread and cake without issue, you've got no reason to replace it. KitchenAid stand mixers are built like tanks, and the older models are arguably more durable than some of the newer stuff. The core job - mixing dough and batter - hasn't changed in decades. Sure, newer models have extra speed settings and fancier attachments, but that's marketing fluff for most home bakers. You don't need 12 speeds when 2-3 do the job. The bowl capacity, motor power, and basic attachments on your 15-year-old mixer are probably still outperforming half the budget alternatives people buy nowadays.
The only real reason to upgrade would be if you're doing something specific the newer models handle better - like if you're making massive batches regularly or want a dough hook that works differently. Otherwise, keep using what works. These things last 20, 30 years easy if you don't abuse them. Spend that upgrade money on actual ingredients or a better oven if you want to improve your baking. Your mixer's earned its place in your kitchen.