People like to compare things. I've done it myself. Anyway, for your examples of maxing Riven mods vs Soul+Heart and spending stupid amounts of money, what I can say is that in both cases, maxing these only makes already trivialized content even faster to do.
You can add the old versions of Tera to this comparison as well. Back then, you only needed mid tier content to do everything in the game. But there was still the so called vision maker (VM) gear up for grabbing. Getting the materials to craft it and then to max enchant it costed people either a long grind or they could whip a credit card, buy off some stuff from the cash shop and sell it in game for gold to buy mats from players who dedicated themselves solely to farming it. But other than bragging rights, such gear was simply not needed to complete the content. You could use it for speed runs, though.
Now, since I'm not even on mid tier gear for this game, I have to trust on what others can say about how high one must get here to reach a point where at least all pve content can be comfortably completed without needing more dps. Is an awakened soul (the base + 140 atp on a 8 second CD) really needed to reach at least a comfortable point? Is it really THAT useful when you could be trying to get instead the much cheaper to achieve boss attack power? After all, dps is needed mostly for boss fights. So stacking that stat can be extremely useful. Then again, plain atp serves both for pve and pvp, and we should always remember that this game has a pvp element where you earn materials not obtained through pve. Heck, some may argue the whole point of maxing is to trivialize pvp. So there's also that. Still, how much is really enough?