It's not just "If Mena turns into anything special" to me, it's the philosophy of the deal itself. This is where Hahn and Williams went haywire. Ok, one of like 47 places they screwed up, but a big one.
For a long, long, long time it was "oh it's ok, we can make a deal like this, we give guys up and they never turn into anything." it's ok to give up Poreda, Richard, Sweeney, Olivo, Hudson, Holmberg, De Los Santos, Masset - no one there was a franchise killing player, so it was constantly ok to give up guys to supplement the big league roster - for years.
Then all of a sudden, in the space of 2 years, the White Sox gave up Semien, Bassitt, and Tatis (Montas could get thrown here too). Any one of those first deals you could say "oh this is fine, we didn't give up a guy who killed us and we got a tolerable baseball player out of it so it was a win, no problem let's keep doing this". Then, it imploded and wrecked the franchise for years.
This one might work out fine, but you do this type of deal enough and you're playing with fire. The odds of any particular deal blowing up horribly are low, but if you do enough deals of the same sort, they can.