Well, if anyone fails a speech check, they can just reload too. Like the combat example, you will only survive if you have healing items, you also have speech increasing items for those occasions (magazines, increasing CHA, using some equipment, etc).
If you want to be able to pass hard speech checks, you invest in the skill. Otherwise what's the point in it being a skill anyway?
I see the cons and pros of both % and set number. And there is no "best" one. In a more P&P game system, the Skill Check makes a lot of sense, because those systems are specifically made to take into account the character failing and it has much more freedom, since the character is controlled by a real human and the game is not constraint by being in a computer game where limitations abound.
But Fallout New Vegas is too limited to implement a real P&P system, so if you fail a check, the game doesn't allow the same freedom as a tabletop game would, because there is only so much you can program.
That is why I always thought that TTW should do what FNV does, make all the speech checks a set value, but then have an optional to change it and have ALL of TTW being %. It is all about the player preference, and this is all about preference, there is not a superior or better way, they are both good and bad.