Using proxies for min/maxing in that way is a bad habit which occurs in most gaming genres from pen&paper to computer games and isn't something one has to support.
X-Wing is a inherently competitive game, same as Chess, Warhammer 40k or Monopoly. Anyone who plays a list that isn't the best it can, or flys it with less skill then they can is not doing the other person a favor, they're doing nothing more then cheapening the win.
If that means I have to proxie a few cards because I won't spend $60 on ships I don't want... So be it. But that is hardly the same thing as saying I'll start to use paper cutouts to represent ships I don't even own. The Slippery Slope argument is considered a logical fallacy for reason.
my point is, that there is no slope slippery or otherwise, the only difference in what you allow as proxies is how much work you have to put in to create them.
In particular when considering your argument of cheapening the win, why stop someone from using a paper model for the models he doesn't have if he can't have otherwise the best list he wants to field?
From my perspective, when you want to allow proxies (for which there might be useful cases) all further limits (upgrades yes but no pilots, or just cards no ships) are arbitrary and very subjective in definition.
it has been said before, that one doesn't want have a pay-to-win scenario, right?
But consider, from the point where there were non-mandatory expansions it has been pay to win, at least if you don't allow proxies for everything. I don't think you can field a competitive list with one x-wing or two tie fighter, regardless how many cards you proxy. (if you think of the starter as the mandatory game)
As I said before, I have no problems with using proxies or not using them (depending on the situation), but I just don't see a difference between ship and card proxies, that artificial line doesn't work for me gameplay-wise.
Sure, for the visual style it's nicer to have the original ships.