Honestly, I may get dismissed as "fanboying" but in MMORPGs and other large scale games I'm pretty sure it's completely possible to change some code somewhere that can adversely effect other things. Maybe the problem is with QA but I truly dont feel every stealth nerf is intentional.
To reply to the reduced drop rates my only really way to judge this is based on Cursed Shore farming pre-bot ban. I've farmed my fair share and I'm under the impression that the reduction in the amount of drops is actually directly related to the amount of people doing the events. More people = Higher level/more mobs for some events I'd assume the higher the level the higher chance it has to drop loot. But, since ANET isn't exactly transparent about how drops/farming/dr work I guess the tinfoil hats can be justified.
Every MMO dev team has the ability to manipulate drop rates. It does not surprise me to see things being adjusted as the game progresses and I feel that it isn't only the result of Anet pushing gem to gold ratio I'm sure that could very well be part of it. However, keeping inflation under control is pretty important IMO. Not to mention the fact is that most Cash Shop driven games are actually supported by a small % of the community. I'm not going to make up internet statistics but I'm sure you can google that and have it verified. Thinking it's just to push gem to gold seems a little silly.
It's entirely possible. The problem is, it's easier to come to that conclusion when you get down to motives. For something like WoW, it's clear that it was done because it would cut short their grind. But with a sub MMO, everyone paid the ticket price, so it only comes down to the time you have and what you do with it. And that comes down the debate of the casuals vs the hardcores. They want everyone to continue to pay the membership fee, so they're trying to find the happiest balance available.
With a cash shop, gold for cash MMO, though, you cannot outright exclude a for-profit motive. They let everyone in the door for free (ostensibly, tho there was a $60 lifetime pass), but now they want you to drive you to the services that make additional money. The games, the food kiosks, etc. You also know where you rank. If you're one of the loiterers, one of the mallrats, all you do is provide scenery. You generate resources, you keep the economy going. If you weren't around, they would probably up drop-rates to keep items from becoming scarce (like in WoW... low level stuff being super-scarce). If you're so reticent about not spending any real cash, then the company has no reason to cater to you beyond keeping the population levels healthy enough to let the game appear vibrant.
TLDR: When a company has a way to convert your RL money into in-game purchasing power, it's hard to exclude that as a motive for any change to the game economy.