Here are the top pitchers by fWAR from 2023-2024 and how they were acquired:
Wheeler: Traded to Mets in '11/ FA '19
S Gray: Traded '17, traded '19, traded '22, FA '24
Z Gallen: Traded '17, traded '19
L Webb: Drafted
Gausman: FA '19, FA '20, FA '21
Z Eflin: Traded '14 (twice), FA '22 (not expensive)
Strider: Drafted
Kirby: Drafted
Lopez: Traded '17, Traded '23
Skubal: Drafted
Cease: Traded '17, Traded '24
Cole: Traded '19, FA '19
Glasnow: Traded '18, Traded '23
A Nola: Drafted/ FA (re-sign) '23
Burnes: Traded '24
Steele: Drafted
Castillo: Traded '16 (twice), Traded '17, Traded '22 (extended)
Monty: Traded '22, Traded '23, FA '24 (1 year deal)
S Lugo: FA '22, FA '23 (moderate deal)
6 out of 20 were acquired by trade in the last 1-2.5 years)
2 more were acquired by trade over 3 years ago
7 were acquired through Free Agency, some not at a high cost or long term.
6 were drafted
Not many traded for were "young," but some were not old, and some are very good pitchers.
8 by trade
7 by FA
5 by draft and holding on
I'd have traded Duran for Castillo and maybe Lopez. Cease? Maybe. Burnes? Not for 1 year. Glasnow? No.