The Hawks were a 20 year old design when the RAAF got them, The P8 is based on a design that has evolved from the original B737 of the 1960's. How old the basic airframe design is, is overshadowed by what is fitted to and into that that basic airframe.
Yes and no, I don’t think it is as simple or black and white as you suggest.
The original Hawk T1 design goes back to the early/mid 1970s, then there was the 100 series, followed by the 120 series (of which the RAAF 127 aircraft are developed from).
From what I understand there were very significant airframe changes/modifications from T1 to 127, they may look very similar externally, but big differences internally.
Equally I’d imagine that a B737 airframe of today is a very different animal to the original B737, the P-8A airframe is different compared to a commercial B737-800NG airframe.
So looking at a 20 yr old T-50 airframe design vs today’s T-7A airframe, I don’t think it’s just the avionics.
Time and technology marches on, even if it’s just simply better manufacturing techniques, and/or taking into account of future ‘tech’ the airframe will be fitted with.
I’m not slagging the T-50, simply pointing out that a 20 year younger airframe design may well be better today as the basis for a trainer that will likely serve up to the 2050s and potentially beyond too.