Their food has gotten noticeably worse compared to their competition but I don't agree that there has been a decline in ground services. They never existed in the first place so I don't see what has declined. But yes, there's no denying SQ's cost cutting is in full effect.
Regarding conducting security at the gate, a tip; while the absence of fast track is not ideal it really isn't a huge issue. If you're a premium passenger just turn up as late as possible. They won't refuse you and why would you want to turn up early? To sit in the holding pen? I fly out of Changi weekly and I never have an issue.
I also don't think TG should be on that list. Mediocre airline with a poor F lounge at a horrendous airport.
It's horses for courses at the end of the day. I'd fly SQ over LH and AF for the service alone, which you touched on.