Interesting. Do you have any examples? Slow descent into spot checks notwithstanding*, did you really see it take over the game? *I'm concerned with what was there in 1e and 2e, not what it might have become in 3e
My impression was that the non-weapon proficiency/skill system was a bunch of vestigial heat and light (and certainly page count in 2e) with out a lot of flame. There weren't any 'spot' style NWPs. Most being niche abilities like 'juggling' or giving you back the horse riding and reading/writing abilities you had before. The weapon/armor/bow-making skills let you fashion normal equipment -- very slowly and with setup equipment with costs you likely never would recoup. Healing (even with Herbalism as well) provided healing at a fraction of the speed of curative magic (and were only efficiently available to classes which already had said magic). Surviving in the wilderness was an amalgam of 5-13 NWPs*, costing 7-15 slots** ; and even then was specifically called out as being a poor replacement for proper equipment planning*** and following maps.
*direction sense, fire building, survival, tracking, and weather sense; possibly also animal handling, animal lore, animal training, fishing, hunting, mountaineering, rope use, and set snare
**more if you want survival for more than 1 of 6 terrain types, potentially adding 10 more slots to the cost
***"The survival skill in no way releases the player characters from the hardship and horrors of being lost in the wilderness. At best it alleviates a small portion of the suffering. The food found is barely adequate, and water is discovered in miniscule amounts. It is still quite possible for a character with survival knowledge to die in the wilderness. Indeed, the little knowledge the character has may lead to.overconfidence and doom"
All in all, my impression was that the system, in the end, didn't really do all that much. It just sat there taking up up page count and making false distinctions between 2e kits and such. In play, to, I remember a whole lot of futzing over picking nwps at character creation (often as a way of theme-definition for a specific character), but then they didn't really see a whole lot of use once actual play started.
All of which is just my take and experience, YMMV, etc.
There were a couple niche actual adventuring abilities in the PHB NWPs.
Picking up a new language in game after figuring out starting ones could be nice.
Tumbling allowed no damage from a 10 foot fall and half from others up to 60' on a check, improving unarmed attack rolls by 2.
Healing was sub par compared to magical healing but was a distinct improvement over default healing and could be a supplement to magical healing.
Mountaineering allowed +10% to climbing chances per time taken.
Blindfighting reduced darkness/invisibility penalties.
Also
Complete Thief's Handbook provided an observation NWP that seemed to be a spot type mechanic in 2e. Also an alertness one to reduce surprise but at the cost of an extra check everytime surprise came up for the chance to modify the surprise check.
Alertness
1 slot, Wisdom, + 1 modifier.
Required: Burglar.
Recommended: All.
A character with this proficiency is able to instinctively notice and recognize signs of a disturbance in the immediate vicinity, reducing by 1 in 6 the character's chance of being surprised whenever he makes a successful proficiency check.
Observation
1 slot, Intelligence, 0 modifier.
Required: Beggar, Cutpurse, Investigator, Spy, Swindler, Troubleshooter.
Recommended: Assassin, Bounty Hunter, Burgler, Fence, Smuggler.
Characters with this proficiency have cultivated exceptionally acute powers of observation. The DM may ask for a proficiency check (or secretly roll it himself) anytime there is something subtly askew; he may also allow characters with observation to increase their chance of finding secret or concealed doors by 1 in 6. The proficiency covers all the senses.
Example: Julina is questioning a man who claims to be a craftsman who has worked on the palace; she is searching for the most discreet entrance. The DM secretly rolls an observation proficiency check; it is successful. "You notice," he tells her, "that his hands are in beautiful condition, entirely lacking callouses." From this observation, Julina may deduce that the man is actually just posing as a craftsman; he may be a con man taking advantage of a few free drinks or coins, or he could even be a spy for her enemies.