Undoubtedly this is heat creep, but the big question is why the 40mm fan with the duct on the heat sink didn’t fix it for you.
As far as the thing working fine new, and then going bad gradually, that’s exactly how it went with me. It could be attributed to the squirrel-cage fan getting weak over time, except your switch to the 40mm fan and duct would have fixed it instantly in your case, and it didn’t. The other thing that I think happens is that a buildup occurs inside of the filament tube, making it less effective at conducting heat away from the filament. You can see it under magnification.
I also think that if there is no thermal compound between the filament tube and the heatsink, there ought to be. I have not disassembled mine to the extent necessary to inspect for and add that, but I think it would make a huge difference in conducting heat away from the filament. Who knows, maybe it is there from the factory.
As far as giving up on PLA, there are just too many reasons not to do that. Lack of warping, compatibility with PVA for support, no fumes, easy to remove from the bed, I could go on… I am a huge fan of PLA and I think it is worth some effort to get it to work.
In my case, going to a duct and fan on the extruder heatsink fixed the problem in 99% of my PLA prints:
https://forum.lulzbot.com/t/dual-extruder-v2-heat-block-fan-upgrade/3366/1
I did have one episode of PLA heat creep since installing the fans, so I decided to also use my extruder body fan when printing PLA:
https://forum.lulzbot.com/t/pla-heat-creep-solved/3236/1
I am not positive that the extruder body fan is having a significant effect, but I think it may be, because it partially fixed the heat creep problem before I installed the ducted fans. Prior to that, I was dead in the water in terms of printing PLA.