Regardless of how you feel about CSS 2.1, it's indisputably better than the table-based layout we had before. Speaking as someone who has implemented both table-based layout and CSS 2.1, it's really unfortunate to see people want to return to the bad old world.
Completely disagree. The Unix philosophy is right when it comes to layout: you want to be simple, fast, and predictable.
This attitude is also why we are in a situation where the Web is slow compared to native platforms. Simple things tend to be fast things. Complex things tend to be slow things.
Your implications that table layout can be "well defined" or "heavily optimized" are both false. The problem is that table layout is ill-defined: it's really a pile of hacks upon hacks that were invented at Netscape a long time ago and still not standardized or consistent between browsers. And, speaking as someone who optimizes layout engines for a living, complexity is the #1 enemy of "heavy optimization". Spec complexity is the #1 reason why layout is so slow, because it makes layout engines large, complex, and brittle.
You can always increase complexity. For instance, by doing all math in Roman Numerals. Thus, in at least some cases you can decrease it. So your statement doesn't hold.
I think you were referring to something like inherent complexity, but then you would have to somehow prove that all this complexity is inherent.
Not really. CSS grid systems (like in Bootstrap) require some ugly and non-semantic markup, but they're not very complex.
If you write your styles in the same preprocessor as the Bootstrap package you chose (Less by default, but optionally Sass), you can mix Bootstraps grid classes into your own semantically meaningful classes, and drop all the col-xs stuff from your markup. Your markup elements can have class names that are purely semenatic. Almost every modern grid system offers preprocessor mixins that can be used in this way.
There are methodologies like BEM that are great for providing a semantically meaningful layer of classes in your markup.
It's true that the complexity remains and is shifted. The important part is that it's no longer interlaced with the complexity that describes the fine-scale structure of the page.
There have been some huge deficiencies in CSS over the years. It is frustrating to have to use complex sets of counterintuitive rules to achieve seemingly simple effects. But there are techniques for helping to manage it in modern web development.
Which is completely not like using CSS at all, right?