"looking back, how stupid is that decision to have Thames in major and to send Snider to the minor. "
Not stupid at all.
If Thames did well in ST, the job was his.
Other than defense, why should Snider get the call?
Success at AAA? Thames had that too.
Success in spring training? Thames had that too - .367 with an OPS of .987 OPS - higher than Snider.
Snider had been given many chances to succeed at the bigs, and while he often started out well, he eventually faltered, and looked ridiculous on breaking balls and change ups.
Thames performed decently last season (offensively), and it was hoped that he could sustain that or even improve on it.
I think they were ready to make the switch right before Snider hurt himself.