You have a good point, but now think about it from the shoes of the person using the unbeatable deck. Because of the 2x weakness, any deck you come up with is going to have a considerable weakness to a certain type of card, and that card, if you run into it, will probably smash your deck to pieces. Are you alright knowing that your new decks now have an automatic new auto-loss added to any possible bad matchups? Does that make it even harder for decks to do well now knowing that they could run into an unfavorable type deck and lose quickly to it?
I remember back to a cities a few years ago. I was running a Empoleon deck with Mantine/Mantyke. During swiss, I run into a Electrode/Gallade/Kingler delta deck that for the most part wasn't the best idea but because he had electrode, the weakness hurt me so bad. He was my only loss. Top cut comes, and sure enough I am playing the same guy. I start with Mantyke, he starts with Voltorb. Turn one, he kills my mantyke. 2nd game, same thing. It wasn't even a game because of my horrible weakness and him being able to KO my basic before I could do anything.
In the past, 2x weakness did have a large impact on the format, but pokemon weren't doing the large amounts of damage they are doing now. Like you mention, at a certain damage amount, the 2x weakness turns into much more damage than +30 ever would. Add to the fact that the newer pokemon are doing more and more damage for lesser and lesser amounts of energy and you have a format where you can lose turn 2 really easily if your opponent has your weakness. Does this improve games? Does 2x weakness help show the better player or just the luckier player?
I don't like it, especially not on basics (that evolve) and stage 1s (that evolve). Final stages, I am 'okay' with. All I know is that I'm going to stick with the Pre-HGSS pokemon as main attackers as long as I can. I cherish +30 weakness over 2x weakness. At least a shot of 70 won't OHKO my entire deck.