I'm still standing by what I've said all season: if you're getting donked consistently, you're doing something wrong.
So far, this is the most true statement I have seen in this entire thread. I've looked back at a literal crap ton of games that I've played this season, and I think I've only truly been donked once. (Kingdra donk, his god hand versus my call-less, supporter-less hand, and this was at a FALL BR.)
Now, I can understand where a ton of you are coming from, having probably been donked quite a number of times over the season, and I feel your pain. The luck of this game is brutal sometimes (This coming from a guy who goes 0-4 on SSU flips on a regular basis), and donks DO happen to all of us. But truly, are the ones making the donks (as auto-pilot decks only) the ones winning? You see some people going around claiming they're going to win by donking every opponent they come by (I think there was a video on someone doing that and his group had under .500 records) and it just doesn't happen. I will admit I have played some decks and have donked some opponents using said decks (I did it with Champ at a cities, I did it with Gengar at regionals), however I knew these donks would cease to exist towards the higher tables where people played much more consistent decks that could set up before donks existed, so I knew I had to have a late game, and lo and behold, I did. I went 7-2 at the cities, losing in Top 2 to Chris Fulop in very close games (And only one of those games in Top cut did I donk anybody out of like.. 6-7 games), and 7-2 at Regionals with losses to SP decks that also were very close fights. Honestly, if you are losing consistently to donks and you aren't DOING anything about it (Increasing the probability of getting pokemon, improving the consistency and speed of your decks, etc), you will continue to get donked. I haven't seen this problem and not to be arrogant, I don't think I will see the problem carrying into next season (seeing as how Kingdra is rare now, Mother Gengar got shut up at nationals, and Machamp just plain out sucks) anyways.
Onto the SP situation. I think the SP decks are a load of fun, personally. Its a more amusing sort of mirror match which requires a ton of skill to play and remains fun to play while at it, and I would not be complaining if they were to continue, nor would I complain if they were to abruptly end it either (Personally, I'm drooling over Advent Charizard here...), but I'm not seeing the problems with it. Its not forcing a dozen people to play the deck, it will have a psuedo-answer to it in Supreme Victors (Relicanth), and other decks are still going to remain viable despite the SP power creep. Also, I doubt SP's will exist in HG/SS, at least not immediatly. They'll probably do what they did in the DP arc and start fresh. I'd expect to see a Meganium, Typhlosion and Feraligatr LX before anything, and they certainly will not be SP. Then move on to legends in later sets.
So now I'll answer the OP's question. Is the TCG going downhill? Hex no. With numbers that massive and a load of people at nationals having fun, I'm not seeing the "downhill" of this game. Some people might have a few bad experiences with whats gone on this format, and a lot of complaining has ensued, but is anyone really going to leave from it? TCG's are like this no matter where you look. This game is by far one of the most fun and most respectable CG's I've ever had the opportunity to play and some minor thing (that seldom if ever happens to me) will not prevent me from continuing. I'm sorry if it happens to you, but all you can do is suck it up and move on.