A 60% win percentage means that your opponent has a 40% win percentage. Always.
You're assuming:
1) the other player will be running sable-donk as well.
if they aren't running sableye:
2) that the other player will be able to setup magnificently in one turn
if the opponent cannot setup well in one turn, if they didn't start with a collector in their opening hand + draw for the turn, they will still get donked next turn.
If games come down to sabledonk vs. sabledonk, then the flip of the coin wins the game, not this 60/40 stuff. Other than that, sabledonk doesn't have to always go first to win, they just have to hope the opponent can't create an unwinable scenario leading into their second turn.
The way I see it:
Opp. Sableye start, searching for collector doesn't put the basics on your bench. You can still get benched and donked.
Spiritomb start -- well i don't need to continue here because that shuts down the whole sableye mess before it starts (i would think). Let's assume it's poke-body isn't working. The opp. evolves a basic to stage 1, but it's been shown that sabledonk can KO 3 80HP pokemon in the same turn (1 by forcing the opp. to pick it back up with seeker) and most stage 1s are 80HP, so I don't see an issue.
Rare candy is different going into Nationals. The opp. can't rare candy to a stage 2 to survive the donk going into the 2nd turn. They can't even rare candy into a stage 1 anymore (ever). BTS may change things. But the opponent has to spend resources getting the evolutions turn 1 instead of more basics (unless they can get both).
It'd be interesting to see quick ball, great ball, unown r, uxie being the new engine for all non-donk decks in hopes of filling the bench on the first turn.