When is a cart past its prime?

I have a Grado Reference Platinum mounted on a Technics SL-1200 Mk5 that I've been using for almost three years now. I feel like it doesn't sing like it used to. I've given it a lot of attention; aligning it till I was blue in the face, cleaning the stylus every record, dialing in minute changes in VTF/VTA. I feel like I'm doing everything to wring every last drop of performance out of it yet my older (but possibly less used) Grado Green trounces it. I've switched the two back and forth from the Technics to my Rega P2 and in both set-ups the Green edges out the platinum (less so with the Platinum on the Technics). It was not the case when I first got the Platinum. The Green is doing a great job of detail retrieval and delineating voices/musical lines where the Platinum falls flat now. I was blown away at first by the Platinum's dynamics and now they're somewhat m.i.a. Perhaps I'm lamenting a little bit the passing of a great cartridge but is there any hope?
Grado does some things well, and some things not so well. The first to go an an older cartridge is the low end....the suspension stiffens.
Your second sentence answers your question. Your third sentence confirms it.

When is a cart past its prime?
I guess when the wheels won't roll any longer.:)
Well, for what it's worth, Grado does offer very liberal trade in value on your old cartridge. You can send in the old one and get a replacement for about half price.
I've been looking into having it re-tipped/rebuilt by Soundsmith. I like the idea of having a line-contact stylus, especially with how fanatical I am becoming about analog. I want to get the most out of the work I have put into cleaning my records and tweaking my system. The prices are pretty reasonable to boot. It seems like there's a good chance of getting my cartridge back sounding better than when I first got it.
For what its worth, I noticed my new Grado Gold does not have the impact of a lowly Black I compared it to. I think the "better" pricer grados can have less impact than the cheaper carts with larger stylus tip and simpler cantilever. I believe Neil Levinson, audio reviewer for Fanfare magazine many years ago preferred wider stylus tips such as the simpler elliptical and conical styli shapes.

Also, larger stylus footprint tracks older, wider-groove LPs better than fine-line types. In my opinion, Grados are very stable over time, they do not have a rubber suspension that degrades, although suspension may still soften after use-- so you may just prefer the cheaper Grados in your system.