Resistance doesn't do nothing. It raises all defenses (including physical) at a higher rate than other stats. That makes it the least useful stat in the game, but it doesn't do nothing.
Scaling is most definitely observable. Here's how you do it. Hit select on the item description screen and read the stuff about scaling. That'll give you a general idea about what you're looking for. Now here's the important part: When you go to level up, before investing points look at your stats. The game shows you what your new attack rating is going to be for the weapon you have equipped, if you invest in that stat.
The game gives you all the information you need to know that improving your endurance is probably a pretty good fucking idea. It tells you that vitality is for hitpoints. If you can't figure out that hitpoints are a safe bet, you don't belong playing Dark Souls.
A seasoned pro can absolutely beat the game with any build, but a new player in the vast majority of cases cannot.
This is ridiculous, of course they can. Just because people sometimes decide to start over because they change their mind about their build doesn't mean couldn't have done it otherwise. Though yeah, if you are so oblivious that you raise resistance to 99 or something, then you probably won't be the game but it will be because you're a moron not because you leveled resistance.
Take a warrior who does the really weird plan of raising every single stat to 20. That puts him at SL 78. About that time, if this person is dying all the time because of the low HP and Endurance, and somehow didn't figure out that dumping all those points into Attunement and Resistance were questionable plans of action, they can raise their VIT and END to 30 each before SL100. 20/20 Dex and Strength gives you a ton of weapons to choose from. You can upgrade along elemental paths to make up for the lack of scaling-based damage. You can supplement your melee with a wide variety of spells, and you can have healing to help you make it through new areas. You can also, of course, keep leveling up to 120 or even higher based on what you think you need.
If your goal is only to beat the game, almost no build is truly unrecoverable. Hell, given enough time, literally no build is unrecoverable. If your goal is to build an optimized, competitive PvP character for the popular PvP zones, well yeah you might just have to accept that you're not going to get it right on your first try.