I'd like to dig a bit deeper and ask: when do we draw the line between streamlining and dumbing down?
On the surface both operates on very similar principles: the removal of unneeded elements from a product to create something that is easier to understand and operate, but the results are pretty much polar opposites. A lot of people, especially the most grognard, seem to assume that they are one and the same and they scoff both at the same time. Yet simpler doesn't mean dumber, in actual fact it usually means smarter, if you can get to the same result with less effort and time it's usually considered a good thing. I think that the key difference is that streamlining works by finding features that are redundant and removing them, whereas dumbing down means chopping down feature arbitrarily.
For example (and I'm sorry to pick on Bethesda again) was removing the character stats from Skyrim a streamlining or a dumbing down?
I think it was a bit of both, on one hand it made the character screen simpler, by removing the stats it made the character progression more linear and easier to handle; on the other it had an impressive domino effect that changed several elements of gameplay significantly: carry weight become a fixed value, quests improved skill directly and so on. In the end what we got was a game that was easier to understand, but also felt more artificial in so many ways.
For a different perspective let's look at the character system of the Witcher series. For all intent and purposes this is a pure perk system not unlike Fallout 4, but I've not heard anyone complaining about it. Why? It's possible that is more a matter of tradition (i.e. we never had a Witcher game that gave us unlimited freedom in creating our character), but I think it's more a problem of focus. In Elder Scrolls and Fallout games there's a lot of focus on creating your character exactly as you want. Fine tuning skills, stats, perks and equipment is a major part of the enjoyment, while in the Witcher games fine tuning Geralt is more of a secondary element. This is the second point in differentiating the two: how does those changes affect the focus of the game. This is, of course, strictly linked to the Bartle's Taxonomy of players (see the video below), so what one might perceive as a streamlining because it rimmed down an element of the game he didn't like or used (e.g. I rarely play a wizard so the lack of a spellcrafting system in Skyrim was hardly a problem for me) another might consider dumbing down because it touched exactly what he enjoyed.
One third element is how these changes affect the key elements of the gameplay, i.e. where is the focus of the game? In a game that focus on exploration adding super detailed map markers and expecting the players to use them all the time is a dumbing down because it means that the main focus of the game is now less prominent and enjoyable. In game based on interactions with NPCs map markers are very much an afterthought, but chopping down dialogue choices isn't.