Pedantic
Legend
The treadmill effect has to do with the play loop encouraged by having level scaled DCs, because it encourages DMs to break the relationship between player abilities and skill roll results. Characters don't actually get any better at doing things, the adjectives used to describe the challenges around them just change.That's not really a treadmill. Should equal tier challenges just get easier as you go up in level? It's not like the DC to climb a 20' wall increases. You're just more likely to be doing more epic things as you go up in level, that should have a higher DC. First level PC's climb slippery walls in the rain on an 11+. 15th level PC's climb walls made out of gelatinous cubes on an 11+ and automatically climb over 20' walls. DM's tend to handwave or not spend much time on non-challenges, but those old level 1 challenges like making a fire with damp wood are still there if you want to see your skill progress.
If that's a treadmill, then combat is a treadmill. You guys fight a lot of dire rats at 10th level? IMX, people fight appropriate combat challenges, with the occasional chump fight to show how you've grown.
Your climb check is a great example. The player didn't get a higher climbing speed, or learn how to ignore slippery conditions, or how to climb at full speed on ceilings and so on. The player hasn't gained a capability they can leverage: they won't make any different decisions because they are now an expert climber and can easily cling to ceilings to drop on their foes. Instead, the DM was prompted to take the same challenge "make a climb check with a 60% chance of success to get over this wall" and use different language to describe it.
5e does this too, just with even less different language and slightly more scaling.