Yes - strange as it may seem, after 20+ years (22 for youngest RAAF example, 27 for oldest RAAF example) the airframes are getting a bit fatigued - by the time their replacements arrive (in another 5+ years), they will be close to worn out.
Since the actual timeframe for replacement, even with the process starting now, is more like 10 years before the first deliveries, then we are looking at a fleet average of ~35 years at replacement.
Sure, you could spend over 25% of the cost of a new aircraft for a complete refurbishment of the airframe to get a significant extension of the useful life - for an aircraft that reflects pre-1984* design, maintenance, and operational considerations - unless you spend even more to install more modern avionics etc.
* first flight of first PC-9 built