I'm the one who brought this up on discord (at least this time, I know others have brought it up previously) so I might as well chime in.
I think this is a really good change tbh. Second sendout in TLG has a pretty big advantage right now, since it gives both pokemon counterpick and the ability to answer your opponent's item (which honestly I would argue is even more of an advantage than pokemon counterpick right now unless your opponent messes up their teambuilding or first sendin). Right now if you roll first sendout game 1 and lose, you get first sendout in game 2 as well, meaning that if you get first sendout in game 1 you have to win a game from the worse starting position in order to force a game 3. That being said, I think most people would choose second sendout every time with the TLG meta as it is now, so the more I think about it the less sure I am how necessary this change is, because let's look at what happens before and after the change:
The player who wins game 1 currently gets second sendout in game 2, and if they lose that then they get first sendout in game 3. Then either they need to win game 2 with second sendout or game 3 with first sendout again. However, after this change, they would get first sendout in game 2 and second sendout in game 3 if it happens. Thus, in both cases the player who wins game 1 gets 2 opportunities to win, one with each sendout position. This change effectively just means swapping games 2 and 3 in this case, so while it might change a 2-0 into a 2-1 or vice versa, it shouldn't actually materially change the result.
However, although this change won't materially change the result of the TLG in theory, this doesn't account for player morale or how it feels to be in this position. It feels really bad to be put in a disadvantageous spot game 1 and then, upon losing, be put in a disadvantageous spot again. Consistently playing from a deficit is not fun and, even though there's not actually any material change, it feels a lot better for each player to be guaranteed to start in the advantageous position at least once. Thus, the change might be worth it from a player feel perspective.