I really don’t agree with: get stronger must also have world get more difficult. This is how you get scaling difficulty, where you never feel like you’re going anywhere, because every upgrade is made pointless by enemies just becoming more dangerous in turn.

But in the other case - you get stronger but the world doesn't get stronger around you, say you keep fighting the exact same lame enemies as in the beginning - then why even continue playing?

The greatest challenge was right at the start. Unless the later parts are exceedingly brief, just letting you bow out on a power rush, why have all those later parts that just end up being trivial?
Shouldn't the former difficult enemies at least become the trash and new, tougher, enemies get introduced to uphold the difficulty as your new abilities and upgrades make the old ones trivial?