It's not uncommon for stock input shafts to strips in the hub, even in bone stock trucks too I believe. Although, some reported they push 500+ hp through a stock input shaft for years, while some others breaks them in bone stock trucks driving like a grandpa.
Billet 300m one piece input shafts (stock input shaft is 2-pieces, as you can see here, it's stripped where mine go together) are a big upgrade but they go for ~$7-800.
Also yeah outputs don't always breaks but if it breaks, it can be replaced without tranny removal actually. Many people consider the stock output are preferred over a billet output so it act as a fuse because if it breaks, nothing it'll take out and are easy to replace verses input shaft, flex plate, and some more.
Whatever.
