The limitation is based on the ceiling, where PowerPC hardware was left-off
the upgrade path and Intel-based Macs continue on. You are at the final version
of iTunes, and final OS X system... Plus a few other last step versions in PPC.
I've found a way around that is to get an earlier MacBook for about $250+
with at least Snow Leopard 10.6.8. (MacBook era: 2006 to 2010) The last
models in this series can run Mavericks 10.9.3. Not sure about later OS X.
Or a MacBook Pro. Check specs at everymac.com or download free database
application from mactracker.ca to compare numbers and check upgrade limits.
Good luck! 😐