All the things you can pour your time and energy into and you choose... muscles?
I used to be of the same mind, but there are a lot of reasons to build muscle mass that have nothing to do with peacocking
Resistance to disease and injury
Better insulin resistance and metabolic condition
Lower all-cause mortality
Faster recovery from diseases
That lightness and fleetness of foot you mentioned earlier
All of this feeds forward to mental health
Everything is just easier when you're stronger
And a man of your advanced yers should understand building muscle slows ageing and leaves you less likely to be an impotent, bent and bedbound wreck past your seventies.
2000s and 2010s was the revenge of the nerds era, 2020s onwards, for good or ill, will be the revenge of the chads.